I had mine sent off to Marvin Pitts and he cut a standard 16.XXX" 516 upper down to 10.5". He also opened up the gas port a little. The thing gobbled up everything I threw in it!

 

I still have my money order made out to Marvin, just need to send it all in, but waiting on possibilities of extending the handguard on the 516 first (something like the PWS Mod 1 keymod would rock)...

The typical SBR NFA process works like this:
-You submit a form 1 for SBR along with $200 money order and wait around 6-8 months for approval for your SIG 516 (or desired) AR lower.
-After you receive your tax stamp approval, you get your AR lower engraved with the name of your trust, and then you can chop your barrel to the length you stated in the form 1 application, and you can mate the upper and lower together. Always keep a copy of the tax stamp in your wallet or where ever the weapon resides. You may now mate this lower with any upper with a barrel of less than 16".

Early barrel chop exception: You can chop your barrel before the approval process, BUT the lower and the upper must NOT reside in the same household. If they reside in the same household it is still considered intent to illegally own an NFA item which is a felony punishable by up to 10 years in jail and up to a $100,000 fine ($500,000 for a corporation or a trust). The benefit of owning an AR over an AK is that you can chop the barrel early and still mate it to a pistol AR lower. An AK will require approval first, then a chop. If you chop it before approval, it must reside at the Class III shop where you are having it chopped until the tax stamp is approved.

Now, back to the main subject of this thread (lets try not to derail the subject).

BigBrakes, do you think there's a possibility of utilizing an even longer rail or seeing the possibility of an addition to the current carbine length rail being made? (The only thing I can think of is a barrel clamp, but that contradicts the purpose of a free floating barrel and it conducts too much heat from the barrel)

Darith,

Any longer of a rail would really up the complication level.  In my opinion, the only possible angle of attack would be some serious machine work to the gas block.  You could remove a lot of material below the barrel including getting rid of bayonet lug.  You would then need to contour the sides down some and then figure out how much material to remove on top..  Also, I didn't mention much about this before but the Sig piston is large enough in diameter that you really have to be careful to have a handguard with enough room to allow it to operate freely.  I didn't have an Adams Arms or any other piston on hand to compare measurements but I'm confident Sigs is larger.  

Only other option that I considered was a 2 piece rail that was 12+" with a FSP cutout and then enlarge the front of the FSP cutout to give room and access to the piston.  This option would force you to remove the rail to remove the piston.  And again, you need to make sure the chosen rail offers enough room for free movement of the piston.  So changing out barrel nut and significant mod of the handguard would be necessary.  If I had more money to throw at this project I would consider trying this.

BigBrakes76

Understood, the work you've done with the Centurion rail is phenomenal as you are the first to go this far with the 516.

How about something along the lines of this rail extension for the SIG 516?

Ahhh, I see what you're talking about.  I suppose that could be possible but would require A LOT of work.  Plus for me, the added weight to the front would counter out the advantages of the added real estate.  

That is a good looking rifle.  Buddy has exact same set up in black.  Definitely want one in the future.

BigBrakes76

Amazing

Just a couple of updates....

In between the last range session and today's range time I have made a few minor changes to the 516.  I have upgraded to a set of Troy Tritium folding sights.  I changed the furniture to all black.  Still need to upgrade the MOE stock to at least an CTR but I'm ok with it for now.
Also, this was the first time I took it to the range in the S.O. Tech Go-Bag Extended.  I had been waiting for the internal divider to come it.  I love the bag.  It fits the rifle perfect and I had 8 mags in the front pouches today but could have easily carried probably 10-12 or more.

I had a couple of friends with me and of course they wanted to fire the rifle today.  Again, it was 55gr XM193.  We fired at least 500 round through the rifle today without a single issue!  Not that I expected one, but it is really solidifying my confidence in the rifle.  Sorry I didn't take pics at the range but I forgot to take the camera and I leave the phone behind on range days so that I'm not bothered.  
I did clean the rifle again because I'm OCD but I didn't really need to.  That's the one thing I really love about the piston rifles.

Anyway, here are a few updated pics.  

Rifle with its new sights and furniture.  Now a VERY good looking rifle in my opinion!



Taken down and going into the bag.




In the bag and 8 mags loaded but could have fit more easily.



Post range carnage!  3 people going nuts with my 516 led to this.  Need to make a new target frame now.  A LOT of rounds down range today.



BigBrakes76

Considering the disappointments with the lack of finish durability, is there another rail you now think you might would go with, if you were to do this project again? OR would simply strip & refinish the Centurion be the easiest solution?  I'm particularly interested in controlling the heat issue at the gas block as you did; I have a "gun guru" with a machine shop as a best friend with lots of experience creating parts from scatch .   My Sig is a Gen4...

As far as the finish issues with my Centurion rail, I'm unsure if that is the norm or possibly if it was just a quality defect with mine and maybe prep was rushed and the finish suffered for it.  It's still a quality rail and I would consider using again for a similar project but, yes, I would consider some cerekote.

I think this Samson rail would also be a great prospect.  Samson

It's the rail used on the AA rifles.

BigBrakes76

Look at what I found:

Rail extension

Very interesting.

Definitely interesting.  I still like my mod better.

BigBrakes76