Over the last few years since the Sig 516 came out I've had a love-hate relationship with it.  I had a Gen 1 when they first hit the shelf.  Loved it at first but the heat near the gas block was an issue for me and the proprietary nature of the piston system bugged me.  In addition to those problems was the inconsistency of configurations with the rifle as it was hitting the shelves.  Different accessories like red dot sights, folding sights etc.  I saw different items that the rifle would come with when purchased as well as subtle differences in the rifles construction like differences is gas blocks when new owners would post up pics.  It seemed to me that details were still being worked out and that worried me for the future life of the rifle.  If something broke, could I get parts etc.  I also wasn't very happy with the Sig rail system and liked the idea of upgrading it to a rail that could wrap the gas block hoping to help with the heat issues as well as personal preference.  I discovered that the barrel nut was unique and when I contacted Sig they told me they had barrel nut wrenches made specifically for the nut but would not make them available for purchase.  All this added up and I figured I would sell the rifle to a buddy that was just getting into ARs for a good deal.  Sold it and moved on to something else.

It wasnt long after this that I started seeing Gen 2 516s on the shelves with the different gas block and piston.  To me it confirmed what I had thought about Sig releasing the rifle before sorting it all out.  Not even gonna mention the legal issues with the carrier.  I eyed the Gen 2 but wasnt ready to pick one up yet.  Just recently I picked one up in trade and figured I'd give it another try.  There are small things that I like better about the Gen 2 like the getting rid of the white lettering on the lower for one.  The Gen 2 also seems to balance better but without them side to side thats a guess.  Anyway, I shot it a bit and was immediately reminded about how hot the gas block gets.  May just be me or how I use the rifle, but it seems to get much hotter up front than any other AR I've ever owned which is a lot.  Decided to try to trade or sell the rifle on the EE and guessing things have taken a downward swing and didnt really get a single serious offer.  It was at this point that I decided to put some of the ideas Ive had floating around in my head to reality and do something about some of the issues I didnt like about the 516.  Basically make it a keeper by making it what I like and want as well.

So what will follow in this thread is my project with the 516.  Im currently on "Stage 1" which was the idea phase and the disassembly phase.  ***I realize that great companies like ADCO are more than capable of doing the work I am going to do and probably more effeciently but whats the fun in that!!!  As of right now Im thinking of the following changes/upgrades to keep it fairly simple and affordable.  Barrel will be shortened to 14.5" and flash hider pinned.  Barrel nut will change and changes/mods should allow the use of different rail systems.  This is the most "unknown" of the project right now but I have plenty of good ideas here.  Trial and error after barrel nut removal.

I will very clearly say that Sig does not want you to be able to disassembe this rifle easily or like any other AR.  You will see in one of my below pictures the first stage of my barrel nut tool (just first rough step) Ill have more pics along the way.  A standard barrel nut wrench is to big to remove the Sig barrel nut.  Ill see how mine works and modify as necessary.  As you can see, I am making more than 1.  If the tool works well I will make the others available to those that need them down the road.  Hoping to have the barrel nut wrench done tomorrow and then test.  Ill post how it goes.

The other issue with disassembly is the gas block.  Holy Shit that things on there tight.  If you attempt to remove it in your garage or wherever you do your AR projects, it will be very easy to mar the shit out of either your rifle, furniture, tools etc.  This is not the place to be using cheap shit tools that get you by on budget builds.  The taper pins are IN THERE!.  No way was I going to chopping at them with a hammer and punch.  There are better methods and thankfully I have the required tools and equipment available.  After removing the pins very successfully without a scratch I evaluated the gas block.  I gave it a light love tap with a mallet and quickly realized that the gas block is pressed into place.  So again, no chopping, whacking etc.  I used the "right" tools for the job and very carefully removed the gas block.  Its a tight fit all the way to the taper in the barrel.  Only then does it finally free up and slide off.  You'll see in the below pics of the barrel with gas block removed just how tight the fit was.

Anyway, thats where I am for now.  Gas block off.  Barrel nut wrench just about done.  Hope to have barrel nut off tomorrow without incident.  Ill report how that goes.  Then Ill be doing some thinking and planning on the next phase.

First pics to show some current progress.








 

I have a gen 1 and would love to change some things too. I'm anxiously awaiting the rest of this report. Much appreciated.

Barrel nut off without any issue today.  Easier than I had expected but still more difficult than any other AR I've worked on.  The wrench was a winner for sure.  Pictured below is set up I used to remove barrel nut.  Thankfully there was no thread adhesive etc causing pain in removing the barrel nut.  Just need a good wrench, a good breaker bar, a quality vise and some elbow grease.

Only finished the wrenches and removed the barrel nut today.  Tomorrow I plan on working some barrel nut ideas to allow for other rail options as well as starting some prep for shortening and threading the barrel.  Thinking about honing the gas block for easier but still interference fit. We'll see.

*****If anyone wants to inquire about the barrel nut wrench please message me.

You're the first person I've know to be able to successfully remove that gas block. People I've spoken to said it was press-fitted and was pretty much impossible to remove without damaging something. Great job! I'll be following this thread closely. I have a SIG516 Gen2 in FDE.

Yeah, I don't know why Sig does not sell these parts and the wrench. I think its a dumb move because I think that some people don't buy the rifles for that reason. And because they could make money selling parts too. I have a gen 1 and I'm happy with it but the OCD in me would buy the spare parts and wrench just for the heck of it for some future emergency that will never come. More money in their pocket if the sold them. And heck, I might get bored and change out the gas block and piston to a gen 2 if they sold it.

Eventually I bet some after market manufacturer will make the wrench and parts and they will make money that Sig could have made.

Sorry, not much to update today. Busy with some traveling today and tomorrow. Compared barrel nuts today and have some ideas.  I did some quick mock ups using both barrel nuts.  Ill post some comparative pics hopefully tomorrow night when back at home.

Removing gas block was a pain and its definitely pressed on.  As mentioned earlier both the pins and block were not easy to remove.  I had to press the gas block off very carefully.  I plane to hone the gas block this weekend so that future installation and removal will be much easier.  

My buddy is powder coating the barrel nut wrenches today.  He wanted a practice project since he just got into it. Ill post pics of those soon as well.

Any ideas on rail options?  Thinking about the Centurion with carbine cut out or maybe a Samson similar to how Adams Arms does it if I can find a Samson rail.  Open to ideas.  

As far as other manufacturers catching up, I've been waiting a long time for options and so far absolutely nothing.  Couldn't wait any longer.

BigBrakes76

Sig doesn't want to make it easy to disassemble or sell the tools for disassembly because they don't want people doing what you're doing, screwing it up, and then trying to get Sig to pay for the fix.

You seem to have a clue of what you're doing, but a lot of people will wrench on stuff, break it, then expect the company to foot the bill for their screwups. Makes sense to me not to sell the tools for the DIYer as that basically gives a green light to allowing people to do their own mods.

As for the gasblock being hot... well that's part of what sucks about piston uppers. The gasblocks get incredibly hot because the gas is vented there and does not cool as it vents in the receiver.

I'll be interested to see what rails you find work with the 516. The barrel nut is sized to allow the oprod to fit over it, so I'm not sure what forearm setup will have a small enough barrel nut to fit the op-rod but will still allow the oprod to fit under the rail itself.

Without getting into my credentials, ill simply say I have more than enough know-how and skill to pull off a project this simple.  And as far as screwing up my Sig rifle, that's the whole concept behind "my rifle". The second I decided to put the upper in a vise I made the conscience decision to void my warranty. My Sig is one of many rifles I have in this platform and therefore disposable to me in the long run should I over do my skills as unlikely as that is.  To me, what I'm doing is on par with those that have trigger work done to their 1911s etc. I'm simply enjoying a cross between my hobby and profession. And I've clearly stated that a project like this isn't for the average garage builder.  I understand your point about people screwing things up out of stupidity and wanting someone else to pay for and fix it.  Democrats come to mind.  And that's a shame but that's not me. I know what I'm doing and I would never expect Sig to fix my rifle.

With the vast variety of ARs I've owned in both DI and piston configurations I understand why my rifle gets hot where. Out of the piston rifles I've owned (POF, LWRC, Adams, LMT and the 2 Sigs) the Sigs are hands down hotter and I'm not talking about actual measured temp. I'm talking about felt heat with how I hold and use the rifle. Again, something I defined as purely my opinion in my opening post. It's my opinion that there is a better option for me by changing handguards.

Lastly, I've taken multiple and incredibly detailed measurements of both the Sig and standard barrel nuts. For the person who doesn't deal in numbers that far right of the decimal point, the barrel nuts are the same diameter. Only difference is in depth and of course the splines on the nut. Where the op rod functions and rides across the top of the barrel nut an idiot with a file and dremel could flatten the splines of a standard barrel nut and actually make it work just fine with Sigs system. And as far as the handguards, mods to any # of handguards is a simple option (for me ) but for now I'm keeping it simple and sticking with a design that i know will work with little fuss.  I also realize that any of the handguard manufactures out there may not want me messing with their product and in doing so will be voiding their warranties etc. I'm fine with that.

Ultimately there are many things in life that come from the factory in certain configuration for multitudes of reasons. Usually because bean counters and lawyers said so.  But thankfully creative people keep finding ways to squeeze more out of their products. The "aftermarket" business is alive and well in industries that I care about.  

BigBrakes76

**Update/After thought - There's also a reason my photos are very vague and only show some basic details to clarify my descriptions. I do not intend for this to be a "how-to" thread. It's simply me documenting the changes I'm making.

Wasn't trying to come off as a dick. The project sounds very cool, I'll be watching it.

Bravo sir. A longer rail that wraps around and extends beyond the gas block is what I've been looking for since day 1...

The PRI Delta handguard may work. Not sure about clearance for the OP Rod into the receiver, but youll have plenty of clearance past the barrel nut. Which is none of your concern, but PRI's barrel nut might help.

Just an idea.

This was my exact question in a previous thread, good recommendation on a barrel nut to clear the oprod.

Ok, where to start for the day and the info/updates.  I think Ill start with my mods of the gas block.  

I measured the OD of the barrel where gas block mounts.  It was measured at exactly .750 which is a standard diameter.  The ID of the Sig gas block was .748 by my measurement for a total of .002 of interference fit.  I went ahead and slowly and carefully honed the inside of the Sig gas block checking my progress every little bit removed.  Each time I checked the fit of the gas block until I got where I could put the gas block on with the same amount of pressure as any other gas block Ive worked with before.  Basicially I removed approximately .001+ and now the gas block will be much easier to work with.  See pics below.

OD of barrel

ID before hone

ID after hone

Surface after hone


Makes doing my mock-ups much easier!


More to come momentarily.

I think Ill cover the differences between the Sig barrel nut and the standard barrel nut.

Some quick #s for those that care then the pics for those that prefer the visuals.  ****Some of my #s look slightly different in the pics.  Very difficult to take accurate measurements while holding camera so the pics show slight less accuracy**** Of course I put this statement for those that have nothing better to do than critique my stuff.  No shortage of those types!!!!  If you are that type of A-hole, ignore any typo's as well!!!

Sig barrel nut OD = 1.430 (I used a scale for my pics but measured with both)


Standard barrel nut OD = 1.360


Difference = .07
 



Sig spline OD = 1.430


Standard spline OD = 1.557


Difference = .127

Length or depth of barrel nut depending on how you're looking at it.
Sig barrel nut = 1.440


Standard barrel nut = .850


Difference = .59


The next photos show the difference when barrel nuts are on and how it impacts the op rod.

Sig first.


Then the standard barrel nut.  As you can see, the splines definitely get in the way.



***WANTED TO BE VERY CLEAR HERE - The rail options that bolt up to a standard AR barrel nut WILL NOT bolt up to the Sig barrel nut due to difference in circumference.  And changing barrel nuts will require removal of Sig gas block and barrel nut.  

Ok, the last of my pics for the day are going to show some things to consider when it comes to the selection of new handguards.  I took several different pics showing how the op rod fits and functions in the current Sig handguards.  Obviously the op rod is larger in diameter than a standard gas tube and one of the main factors in my selection of a new rail will be based on a handguard that has enough room for function of the op rod.  I have the capability and ability to do some machining here and could remove material from a number of handguards but I'm not really looking to invest that kind of work into this.  

Here are some pics to show you what I'm talking about.





This last pic shows the groove in the upper portion of the Sig handguard.  


So to sum up what I want to do here....and trust me, over the past few days I've considered just about every railed handguard out there and how I could make it work.  I even have the capability to manufacture my own barrel nut to make things work but I'm trying to stick with slightly modifying a readily available barrel nut.  Really there are so many good rails that attach to a standard barrel nut that I'm really leaning this way.   I can modify in any way, any of the current manufactures barrel nuts and handguards,  But as mentioned I'm not that invested for that kind of work.
Another consideration is the gas block up front.  Part of my goal all along was to have rail wrap around the gas block on the sides for sure and partially on the bottom.  Again, I could take any handguard and modify the top section of rail to make room for the Sig gas block but there are plenty of handguards already out there that are designed this way.  I didn't want any handguard in front of the gas block adjustment knob to impede function or ability to remove for maintenance.
Last consideration is that I want to be able to remove the handguards or at least a portion of the handguards for cleaning and maintenance.  

Actually the last consideration with all of this is $$$ as well and I mean money as in cash as well as time.  

When you take all this and add it up I have come to a couple railed handguard options that fit my bill.  The Centurion C4 with carbine FSP cutout.  The DD RIS II FSP and the Samson STAR C-EX-P.  TRUST ME, I know there are more options and I could get really carried away which I usually do but for this time I'm trying to keep it fairly simple.  WHen considering these rail options the basic deciding factors come into place.  Like cost, weight, availability, appearance, function, reputation and personal preference.  I'm personally leaning towards the Centurion rail right now.  Going to think on that a bit over the weekend and then make my call.  I really think this rail will accomplish what I want with the least amount of fuss and work.  

As you see in the below pic of the Centurion, there is some material in the top section of the rail that will mount above the barrel nut and would get in the way of the op rod but I'm willing to put the upper section of rail on the mill and make a quick pass to free up that space.  



Also, I think the Centurion will provide the necessary room for the gas block.


Definitely still listening to ideas on all of this.  So review my info and posts and let me know.  I have to admit its been a busy few days and the brain is a little fried.  

If anyone out there has the Centurion C4 with carbine FSP cutout I would love to talk to you and get a few measurements before I order.  Please message or email me if you do and Ill tell you the #s I need.

Also upcoming mod will be the cutting of the barrel to 14.5" and pinning a Smith Vortex.  Need to have some time for this so not sure if that will be this week or next.

Thanks to those watching,
BigBrakes76

Appreciate all ideas here.  I like the weight and ergonomics of the PRI but it doesnt wrap the gas block at all which is one of my main goals with this project.

BigBrakes76

I really like what I'm seeing here. Keep it coming. Us SIG516 owners appreciate this.

I like the looks of the C4 better, but I was reading the Samson STAR C-EX-P is piston compatible straight out of the box. Not sure if that makes a difference.

Edit:

Also, do you know what the exact diameter of the gas port on top of the barrel needs to be to cycle the rifle reliably when you cut the barrel down to 14.5”? I know a guy on here had to change the diameter of the hole when he cut the barrel down to 10".

You're right that the Samson would be closer to immediate bolt up and ready to go.  It's upper groove/channel seems larger and doesn't have anything to remove best as I can tell. But it's a lot heavier and just doesn't seem as overall appealing to me. But for those without access to a full machine shop and equipment, it may be a better option. I wish the wife would let me purchase several of the options I've been eyeing. The Samson's also seem a little harder to come by right now.  Not sure if that's because of their lawsuit and bankruptcy or what.  Optics planet had them. Only place I saw.

I really like the DD rails. If the RIS II MK18 FSP was black it would prob be my first choice. Mounting would require some serious mods that most can't do easily but I think it would be awesome.  The barrel nut would require some work for op rod to fit and then the mounting nut that seats against receiver would need some mods. Only way around modding that nut would be to mod op rod and shorten that back black piece that is pinned to op rod and seats against receiver.  Appealing and I even accept the work as a challenge but its getting riskier and more $$$$.

Now that I've honed the gas block, I'm confident I could even get the DD lite FSP one piece rail to work.

Like I said, I've been agonizing over this and have changed my mind several times.  I've researched to the point of driving my wife nuts.  I just told her I was researching a better CHF'd barrel for this project and I got the "look".  

***Side note:  Aaron556,  I see you're from OH, I'm born and raised in Cincy.  Hasn't been home for a long time now but I miss it a lot.  Bengals flag hangin in the garage.  Don't judge me the rest of you!  

BigBrakes76

Awesome. I'm about 20 miles north of Cincy and watching the Reds game right now! Originally from Fairfield.

Hopefully you'll offer your "services" to folks who aren't mechanically inclined like myself. I'm still astonished you got the gas block off.

The have lengths up to 15". But Im not sure if the triangular shape will fit around your gas block. Maybe the round one might. But I dont have either one to look at, only going by pictures. It being carbon fiber, you can customize it how you like. Voiding warranties of course, but for the benefit of making it yours.

Also, Diamondhead USA has a handguard that might work. They have extended lengths, and shape looks like it would give added protection on the underside near the gas block. No idea on their barrel nut though.

Just tossing ideas out there. I think what your doing is cool, its nice to see people making things their own. Not afraid to scuff up their gear for the sake of personalization.

I have zero experience with the Ergo rail but I think this rail would work as well for the criteria I have.  Think I'm still sticking with the Centurion but this would be a good option I believe.

Ergo Z Float Overshoot Rail

BigBrakes76

I am sooo tag'n this thread.

I want to change out the rail so bad on my 516, but no one makes one to fit b/c Sig decided to be difficult with things. I am looking for a handguard system that has the rail on the top, smooth on the sides and screw on sections similar to the one from Ruger for their piston or one like on the Troy Defense rifle.

With the barrel nut being so long/deep, does it serf a purpose, like a stop for the piston? Could you cut down the spines on a standard nut to fit and have more options for rails?

If you're wanting a handguard similar to a Troy and also wanting it to cover the gas block, that's gonna be an issue as the Sig gas block is very blocky.  Not sure if you've seen the Samson Evo handguards on the Adams uppers but I'm thinking that would be closest to working. But you'd probably have to shave off bayonet lug at a minimum.  

As far as shaving down the splines where the op rod will travel on a standard barrel nut, that's the easiest and most likely way to go about things.  (If you're capable of getting off the gas block and barrel nut)  I may turn down Sigs barrel nut a little. We'll see what ends up working best.  As to why Sig made things the way they did, only they can answer.  It's my opinion that the rail/gas block area is one of the least desirable factors to the 516.  

More to come as I keep working on this project.

BigBrakes76

They were copying Hk... half assed.

Do tell. What did Sig copy from HK?

Lets keep things on track here.  People are fiercely loyal to their brands and that's not at all what my thread is about.  This is simply a customization project and in no way about one brand being better than another or a discussion on company philosophy etc.

I apologize, that was poorly worded. It shouldn't be a surprise to see similar op-rod, and proprietary nut and handguard considering Sig used a former Hk employee. I don't own a Sig 516, but I love this thread! Back on topic...

No apology necessary.  I just want to make sure we stay on track here.  We could discuss for days the benefits of company X's system over Y's.  To me the issue, as I mentioned in the first post, is that everyone's is proprietary.  When asked by friends who want to get their first AR and desire a piston AR I highly encourage them to get a quality DI rifle that meets the specs we've come to demand on ARFCOM and then if they fall in love, think about a piston rifle or upper for their next purchase.  But you are right, back on track....

Did some preliminary prep and set up to cut and thread the barrel tomorrow.  As long as time permits, that's the big thing I want to get done tomorrow.  I did order the Centurion C4 with FSP cutout today.  Feeling good about that.  As soon as it gets in Ill take some immediate pics of how it will fit etc and then make sure it will indeed work.  Ill document any mods necessary to get it to work.
Only other accomplishment today was getting my precision barrel nut wrenches back from the friend who powder coated.  There would be pics but he's not quite the perfectionist that I am and Im thinking I am going to blast the finish off tomorrow and just spray them myself with some rattle can.  He laid the powder on a little thick and that made the pin/spline interface too tight and rubbed the paint off anyway.  I had made the pins fit with a nice tolerance to get some good purchase on the barrel nut and the paint was just a little much.  Anyway, just as a reminder, if anyone needs a wrench for the Sig barrel nut please let me know via message.

If I get the barrel done tomorrow Ill post the pics.

Thanks for following,
BigBrakes76

If this Centurion C4 does workout would it still be necessary to remove the Sig 516 gas block or did you only remove it because you are experimenting? Would you have to remove it to install the Centurion C4 or does it come in two halves?

I don't want to say anything here with super certainty, but I don't think the Centurion will fit around the circumference of the Sig barrel nut as its slightly larger and the Centurion is intended for a standard barrel nut.  

Once I have the rail in I will know for sure exactly what we have going on.  And if that's the case, it will require removal of the barrel nut and in order to remove that the gas block will have to come off.  I'm hoping by the end of the week I have a definitive answer for you and anyone else that cares.

Barrel mods coming up momentarily!

BigBrakes76

Today was cut and thread the barrel day.  To me the Sig has always felt front heavy so I figured getting rid of some unnecessary length of the barrel would really help.  Ill explain what was done with the following pics.

First this just shows that precision measuring tools and drawing were ready to go.



Everything indexed properly off the bore.



Got rid of the old threads and then did a practice run on the end of the barrel prior to doing the final cut, crown and threading.



This just shows some work along the way.  Im not gonna get into all the actual work or technical stuff in this thread.  My pics are just to give you a rough idea of the process.



Test fitted the flash hider along the way as the threads were being cut.



I used precision tools to measure the final length of my barrel.  You will notice that the barrel is a hair over 16.5"  There is a half inch at the base of the barrel that doesnt count towards the overall length as its just part of the barrel extension.  So overall Im right at 16".



Those that know how to read this picture will see what the overall length is.  There is just enough beyond 16" to be safe.



For those that prefer a more simple way to see the length of my barrel I used the good ol super accurate tape measure.





This pic show the 11 degree crown and the perfect fit of barrel/muzzle device.



Very happy with the end product.  My only issue is that while I may be able to do all the work I have demonstrated, I do not have the capability to do any refinishing.  Really want to get into parkerizing but haven't as of yet so for now to prevent any corrosion etc, I quickly sprayed the new threads with some high heat paint.  

Hoping Centurion handguards get here soon so I can test fit and wrap this project up and test fire.

Thanks,
BigBrakes76

Sorry if I'm not quite understanding, but why does the gas block have to be removed FIRST in order to remove the barrel nut SECOND? That Sig barrel nut tool you made is horse-shoe in shape...doesn't it just slip over the barrel and into the grooves of the barrel nut without removing the gas block?

I have a Sig 516 10" CQB upper on order directly through Sig. I'm curious how far beyond the gas block the Centurion C4 rail will extend and if it will cover the flash hider on the 10" barrel. If it does I won't be able to use it.

Answered your own question while I was typing.  

Never mind. The barrel nut has to slipped off the front end of the barrel not the rear. Duh!

Lets get this rail modded!!! You receive it yet?

It shipped from Rainier on the 10th or 11th.  Trust me, I'm watching the mailbox like a hawk!!

Hoping today, maybe tomorrow that it will be here.

Centurion rail came in today.  Have an event to attend tonight so just a few quick notes but ill have pics up tomorrow for sure if not tonight when I get home.

Things I already know from just a quick trial fit...
- Will be ditching the Sig barrel nut.  Won't work with Centurion rail without serious mod.
- Standard barrel nut will need flat milled on area that op rod will travel over but this was known.
- The rail mates up fantastic with Sig gas block!  No need to trim or modify gas block.  Lower section of rail looks perfect with bayonet lug.  Very happy here.  No mod'ing lower rail at all.
- As expected I will need to slightly modify the section of upper rail that mates with barrel nut.  One pass on the mill will fix this.
- One thing that I did not expect, the tabs on upper rail that are for alignment to receiver will not work with Sig receiver.  Pics will help you understand.  The receiver on the Sig tapers down from the rail to barrel attach point.  My other receivers are straight.  Will have to mod receiver for tabs or remove tabs from rail.  Unexpected but very simple fix here.

Plan is to finish up the work tomorrow but I am very happy after the dry fit.  Between shortening the barrel and upgrading the rail it is going to significantly improve things in my opinion.

Ill try to have some pics up later tonight.

BigBrakes76

IMHO, this is the key to the whole project. Looking forward to your next update!

 

Holy sh*t, I'm getting uber anxious to see it now!!!!

Its late but as promised here are a few mock up pics.  Also a couple that show the 2 fit issue areas I mentioned earlier.  Will be getting the rail on tomorrow and hopefully firing a few rounds for ops check by Saturday.

First is the area that I knew would need a little milling.  That will take all of 5 min tomorrow.  As you can see the material on the rail is preventing the op rod from sliding all the way to the receiver.





Here is the other fit issue.  The Sig receiver had to be different as well and prevents the 2 tabs on the rail from fitting properly.  This will also be addressed tomorrow.



Here you can see how nicely the bottom section of rail works with the gas block.  Looks great in my opinion.  This is the view from the under side.  Bayonet lug fits perfect in rail channel.



And finally some mock up pics.  You'll see how the tabs on the rail are preventing the top section of rail from seating all the way down and onto the bottom section.  This wont be the case for long.






Hope to have pics up tomorrow night of the process of finishing the rail and how it all looks assembled.

BigBrakes76

Looking good man. Cant wait for future updates.

Here are some quick pictures that show a few of the details of finishing modifying the rail today.  Next post will be the pics you have all been waiting for!!!

Had the rail in the mill today to get the right fit for the op rod.



Took the material off that I had discussed previously.  Both the material in the upper channel and the alignment tabs that wouldn't match up with the Sig receiver.



And after I attached the barrel nut and found out where the op rod (piston) would be traveling, I took the splines I had mentioned before off so the piston would fit properly over the barrel nut.

Before.



After.

The project is COMPLETE!!!!  (all but the function test at the range that I plan on accomplishing tomorrow morning)  I have test functioned everything and I predict no issues tomorrow.

I am EXTREMELY happy with the results!!!  The feel of the rifle is 100% better.  The balance is great.  And the overall aesthetics are great in my opinion.  Tomorrow will test 100% function as well as the heat issue I had before.

After I do the range session tomorrow Ill put some final #s up on how many hours I have approximately in this project as well as how much $$$ I have in it.

But for now I wanted to get up a bunch of the finished pics for all to see that have been following this project.  

Here are a couple of immediate and overall comparison shots.

Before.



After.




Here are the rest...

Just the upper.  Not bragging here but I'm VERY happy with the overall fit and finish here.  Taking my time paid off.  Looks great!




Gas block fits and looks great in the rail.  Having it partially covered is a HUGE improvement!!









And here is the Go-Bag I plan on using for the 516.  Shortening the barrel really makes the fit perfect when the rifle is broken down.




***Update - I wanted to comment on the Centurion rail real quickly.  The construction of the rail is of good quality.  Not as good as a few others I've tried over the years but much better than the "cheap" brands.  However, I am disappointed in a few details.  First, the finish is very "wimpy".  It takes nothing to expose bare metal.  From just installing and removing the attachment hardware twice it very easily removed the finish especially on sharp edges.  I was EXTREMELY cautious to protect the finish yet it still blemished very easily.  I felt very nervous the whole time I handled the rail and to me that's not the way it should be.  Ultimately I know it's more about function, but for the money spent the finish should be better.
Only other issue with rail is the attachment holes on the sides of the rail.  On one side the hole in the upper section of rail didn't quite match up with the lower section.  The upper slides over the lower and the screw threads into a heli-coil insert on the lower section.  Thankfully I was able to address this issue, but others would have had some problems.  Overall, good rail and very good for my application but if I was choosing a rail for another rifle/build I would choose something else.

Ill have some range and final pics up tomorrow with any final comments.

BigBrakes76

looks much better!!

So, when can I send you mine?

Super windy day at the range today but the mission of testing the rifle went great.  Accuracy was a distant second to running a lot of rounds to make sure the rifle with my mods was good-to-go.  Over 300 rnds fired.  Rifle fired without a single malfunction of any type.  Ammo was standard 55grn XM193 FMJ.  

This picture doesn't really do the wind justice today but was trying to show that you had better be holding on to your paper targets.





These 2 pictures are right after firing 60 rounds as fast as I could.  I could still wrap my hand around the gas block area of the rail.  Only place that was to hot to touch was the front flip up sight.  HUGE improvement over before!!




Accuracy was not my mission today and is not the purpose of this rifle.  My goal today was function testing and then just some general sighting in of my M4s.  You'll see in the next picture that I started low and right and was moving it in.  (Furthest right group was after making the rookie move of turning the adjustment screw the wrong way )
I am shooting with no support @ 50 yards with the 2 MOA dot on center mass.  



The second picture was after changing the paper.  This time I was focusing more on speed than accuracy.  This was me walking back and forth from right to left trying to stay on target.  I was trying to fire the 516 from different positions and angles to make sure there would be no issues.  When I move to my right my shots go left of the center a few times.  When moving left my shots go a little low and right of center.  Accept for the most extreme shots left and the one low and right, I consider that more than completely acceptable for as much as I was moving around and as windy as it was.



Once I fired the weapon moving around and verifying operation.  I fine tuned the M4s just a little more then started taking a few more accurate shots.  I took out a couple of the AR500 prairie dogs I have made and also set up my water bottle and empty Redbull can.  Shot them also unsupported from the same distance but while standing still and had no problem hitting each with first shots.  I acquired some sheets of armored steel (AR500 3/8") a year or so ago and I make myself all kind of different targets.  Here pictured is one of my favorites with a steel circle that flaps but I didn't take it out today.  Just thought you might like to see some of the other stuff I make.  






Then after repeatedly knocking down the prairie dogs I set out a couple golf balls and again had no problem knocking them around a little.  Being able to hit a bouncing golf ball at 50 yards is plenty of accuracy for me.



This is the back of the target when I was done today.  The head shots and the shots to the right of the head shot are from my pictured 229 at 25 yards but all the upper and lower center mass shots are from the 516.  Of course there are a couple of strays from rapid firing, mag dumps trying to choke the rifle and ensure smooth operation considering all the mods etc, but I think it was a pretty good range day considering the wind.  Also pictured is the pile of brass left.  
All brass ejected at the 4 o'clock position.  This has been the same since both before and after shortening the barrel.




These last few pics show what everything looked like after I came home and disassembled.  No issues.  No unusual wear.  Just perfection!!!!





So to sum everything up, I think this project was worth every penny and every bit of effort.  I want to make sure I say that this project is NOT for the average DIY'er who tinkers with his rifle in the garage.  And I would not recommend undertaking any part of the mod without extensive skill and knowledge.   I intentionally left  out a lot of details and a lot of pictures that would show the "magic" of how I did everything.  DO NOT make assumptions or speculations on how I did anything.  

Besides that mandatory disclaimer, this project was a TON of fun and very rewarding.  I am going to be smiling for along time and especially whenever I see anyone at the range with their stock 516s.

As far as time, Id say if I compressed the hours I spent on this project I would estimate 12-15 total man hours.  
My estimated costs...

Centurion rail = $250
Vortex FH = $0 (traded an old Camelbak for it)
Standard AR barrel nut = $10

So I'm happy as is the wife with the money spent on this project.  Well worth every penny.  Obviously I saved a TON of money being able to accomplish all work myself.  If I was to charge someone for my work on a project like this, it would be several hundred $$$$ just for the hours.

I added an update to my post last night about the Centurion rail.  I would like to say that the rail felt very solid and I love how it worked with the 516.  But after the range session today it even more disappointed me with its finish.  It comes off and leaves bare metal VERY easily anywhere there is an edge.  For the money spent that is very disappointing.  I've owned and used Larue, Troy and DD rails and all have far superior finishes that are much better at standing up to the abuse most of us put our rifles through.  The missing finish on the rail pictured is from sliding on my Magpul AFG to hold the rail for alignment and the the missing finish around the fasteners is from installing the fasteners.  
Oh well, if this is my only issue or complaint I'm doing GREAT!!!!  Here are the mandatory pics.





If anyone has any questions or wants any more info don't hesitate to ask.  Ill share what I can.  Thanks to those that have followed and provided motivation.  Future upgrades will include replacing the crappy Sig flip sights, replacing the trigger and upgrading the furniture.

BigBrakes76

Awesome work!!! We shall call this the Gen76. Wish I had your skills and equipment. Ill say it again. Awesome job.

Great thread! Thanks for going into some of the finer details. Makes me wish I hadn't sold my 10.5" 516 upper. I think I am going to be picking up another one up shortly...

You totally should.

I would LOVE to either SBR this rifle or purchase an upper already the length I would want.  Unfortunately, I have no idea how to navigate the SBR process.  

I know NM is friendly but don't know where to start.  I've been told go the trust route and been told to just apply.  Not sure how to proceed.

If I did, I think I would want to go with an 11.5" barrel.  Wondering if I would have to upsize the gas port?

Hmmmmm, got me thinking about things again.

BigBrakes76

I went the NFA Trust route. If I can do it anyone can. It was very easy.

Watch this video and it will walk you through the steps: http://www.youtube.com/watch?v=Z6ONqI3_QOI

Went with a 11.5" barrel on my DI. I know for a FACT you would have to alter the diameter of gas port hole in the barrel if you went that short. Someone on here sent their SIG 516 away to get the barrel chopped down to 10.5" and wrote about it. Wish I could find the original thread.