I hear these questions asked a lot and there is too much misinformation and ignorance around so I thought I'd make a thread discussing the basics of rail length compared to barrel length.

So, what is rail length and why don't they make sense?

The way I measure rail length is by measuring from the front face of the upper receiver to the end of the rail. This is not how all manufacturers measure their rail length. They may measure the length of the rail itself which doesn't mount flush with the upper receiver or some other method. They may also round off the number. I have seen 12.0 rails that actually measure 12.6", that's quite a bit different.

The best way to find out how long a rail is would be to measure it while it is mounted to an upper receiver. If you are shopping for a rail and can't find this information, there are ways to estimate how long a rail actually is. One way to see what length a rail is without measuring it is by counting the slots on the upper rail. You have to make sure to include any that may be missing at the rear end of the rail. Another way is to look at the T markings on the upper rail. You have to be careful with this method however since some manufacturers skip T numbers when marking the top rail. The first slot on a rail (which is sometimes skipped) should be T15.

Some rails are made for specific purposes like fitting behind a carbine length FSB or being flush with the step behind the threads on a barrel. Other rails are just even numbers like 11" and don't really have any reason for being the length they are. Other rails were made to meet specific requirements. The distance from one slot to the other is .394", so rails may come in lengths in increments of .394".

Before we get into what rail length you need, let's get a primer on how barrel length is measured:

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Some people will say "I bought an 11.5" barrel, what rail length do I need?". Just like with rails, barrel lengths aren't always exactly what they are called. As you can see in the image above, a US Military 14.5" barrel actually measures 14.63" when measured using the ATF method. It's not as simple as adding 1/8" to all of the common names either. A 20" US Military barrel actually measures 20.00". Does this mean that your 14.5" barrel measures 14.63"? No. I don't know what your barrel measures. I only know what my barrels measure and what USGI barrels measure. I have personally measured a lot of Colt, some FN, and some Daniel Defense barrels. Other than that, I don't know.

So, the first thing you need to do is verify the actual length of your barrel. Again, the best way is to measure it yourself. You can do this by putting a cleaning rod down the bore, marking it at the muzzle and measuring from the end to the mark. If your muzzle device is pinned and welded; you can mark at the end of the muzzle device, subtract the length of the muzzle device and any shims/washer, and add 5/8" for the threads. You can also measure the entire barrel assembly from end to end and subtract 3/8".

How to use the following images:

I am showing the approximate distance from the front of the receiver to a point on a barrel which measures what it says at the bottom of the image. There is tolerance stack up so this number will vary a little bit. For the length to the rear of the FSB, you obviously should shoot for a little bit shorter than what it shows. Maybe 1/8" or so. Again, you need to measure your barrel to be exact.

If you want your rail to go as close to your muzzle device or sound suppressor as possible, you need to figure out where that point is in comparison to the step before the threads. You may have to have the rail a little shorter or you may be able to have the rail a little longer. For example, the Block II 10.3" RIS II URG has the muzzle tucked under the rail a little bit and it works fine. Every set up is different.

I have shown where the slots of a rail that is close to the length measured would be. The number of slots and the T number for the last full slot are shown.

These are some of the common barrel lengths out there. Your barrel length may not be listed. I don't know every single barrel length that exists, sorry. If it isn't shown, you can probably use the following formula to get what you need:

rail length (measured from the front face of the receiver to the end of the rail) necessary to end flush with the step before the barrel threads = barrel length (ATF measured) - thread length - .175"

Then you can add or subtract whatever you need for your specific muzzle device.

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Tacked and bolded your subject line.  ~d16man

Great post, thanks!  I've needed this in the past and will need it in the future!

Awesome post!
This should cut down on about 1000 posts per year asking what size handguard do I need.

No problem.

Just send them a link to this thread!

This should be stickied and marked as required reading

They'll still ask.

Amazing post
Thank you for your good work!

Good stuff.

Thanks!

Beware of some rails like daniel defense's mfr series that have a bottom lip that juts out further than the rest of the rail.



And great job!

Exactly!

Good point. It could be measured to the top or bottom.

Thanks!

great post sir!

Thanks!

Fantastic!

Maybe next you could compose a thread that explains which manufacturer is the currently the most Toppest Tier?

In all seriousness, this is a great resource like many of your other threads- I’ll be referring to this frequently.

Cool. Hopefully people will reference it before posting a thread.

If I could do it objectively, I would. I'll probably make a similar thread about barrel length and gas system length next.

Thanks!

Very informative thanks for the post.

Thanks!

That would be quite a doozy with all the proprietary systems like the KAC intermediate, Mod 3, LMT MRP and now Geissele’s intermediate in the mix- hopefully that would capture a plethora of frequently fielded questions.  Would be awesome to have it as a trusted sticky for sure!

Fantastic post and love the illustrations! I occasionally still need something like this to reference.

Thanks!

This is pretty damn handy.

Very nice

Got a drawing for a 12" barrel?

No, sorry.

I would suggest you use this formula:

rail length (measured from the front face of the receiver to the end of the rail) necessary to end flush with the step before the barrel threads = barrel length (ATF measured) - thread length - .175"

Hi, OP - great thread!

Can you post up some pictures covering a lot of the shorter 300BLK/Pistol barrel/rail lengths?

In particular these would be very helpful:

8.3" barrel, 8.5" barrel, 9.0" barrel (popular 300BLK lengths)

Concur!

And thank you for this thread, it is incredibly timely.

You're welcome.

I'll look into the pistol barrels but I have no idea on what actual length (ATF measured) they are.

BTT

It's a sticky!  

Just noticed this thread. Excellent post, OP

Do you have any data on Intermediate length gas systems, for those of us who like to "+1" our gas systems on custom guns?  Or are "Intermediate" length systems more of an "anything goes" affair?

There's no industry standard for intermediate but we can definitely figure it out. What brand are you looking at?

Oh, no, I'm not brand specific.  I typically go custom, but I wasn't sure if "intermediate" was a bespoke term - although I did suspect that it was (judging by lack of data on the subject).

Looks like they are all somewhere between 1.5" and 2" shorter than a rifle length gas system. If you get that specific number, then you can figure out what rail length you'll need.

IIRC, White Oak Armament makes custom length gas tubes.

Does anyone know if this size of quad rail is even made or a custom shop that could make it?

Quad? Not that I know of.

I was afraid of that.
The best I have been able to find is a 16.5 inch Quad by Monstrum followed by a 16 inch quad by Tiger Rock

Monstrum 16.5 inch Quad

Tiger Rock 16 inch Quad

I looked on YHM and they have nothing close to a extended anything be it rail, m-lock, or keymod unless you are referring to some long defunct product.

Found a new possibility that I haven't seen. Allows you to have the longest possible rail length while still being able to mount a bayonet to a pinned FSB.

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You can use up to an 8.80" rail on a 14.5" carbine length barrel, 10.80" rail on a 16.5" mid length barrel, and 14.18" rail on a 20" rifle length barrel.

I recently purchased a Geissele Mk8 13” rail for my 13.7” build. But I received a Mk8 13.5” rail. Before purchasing it I had looked online and did find that there is 13” version of the Mk8 rail. When I checked Geissele’s site, I can only find the 13.5” now. Do they still make the 13” version?

They recently transitioned all 13" rails to 13.5" and all 9.5" rails to 9.3".

You might find a retailer with old stock of a 13" or find one on the EE.

Thanks for the reply. I’m a little annoyed because when I ordered it off of Brownells, it was referred to as the 13” version everywhere on the site but the 13.5” is what was sent. I’m using it for my 13.7” build. So I could shim it but I’m not sure if I should really be shimming that much. I measured it with my calipers and I would have to shim it about .21 on an inch.

If you aren't using a suppressor, it would probably be OK.

Someone might trade you their 13" for it.