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Posted: 5/12/2020 3:40:18 PM EST
[Last Edit: 1/22/2021 8:43:21 AM EST by pezboytate]
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:

Attachment Attached File


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.

Attachment Attached File


Attachment Attached File


Attachment Attached File


Attachment Attached File


Attachment Attached File


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Tacked and bolded your subject line.  ~d16man
Link Posted: 5/12/2020 3:53:47 PM EST
Great post, thanks!  I've needed this in the past and will need it in the future!
Link Posted: 5/12/2020 4:18:13 PM EST
Awesome post!
This should cut down on about 1000 posts per year asking what size handguard do I need.
Link Posted: 5/12/2020 4:22:05 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By officerX:
Great post, thanks!  I've needed t­his in the past and will need it in the futur­e!
View Quote


No problem.
Link Posted: 5/12/2020 4:22:31 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By thorn653:
Awesome post!
This should cut d­own on about 1000 posts per year asking what ­size handguard do I need.
View Quote


Just send them a link to this thread!
Link Posted: 5/12/2020 4:28:58 PM EST
This should be stickied and marked as required reading
Link Posted: 5/12/2020 5:33:24 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By firepyro5­15:
This should be stickied and marked­ as required reading 
View Quote


They'll still ask.
Link Posted: 5/12/2020 5:37:41 PM EST
Amazing post
Thank you for your good work!
Link Posted: 5/12/2020 6:01:25 PM EST
Good stuff.
Link Posted: 5/12/2020 6:22:23 PM EST
Originally Posted By Xequals0:
Amazing post
Thank you for your good work!
View Quote

Originally Posted By JohnParker55:
Good stuff.
View Quote


Thanks!
Link Posted: 5/12/2020 6:23:47 PM EST
[Last Edit: 5/12/2020 6:24:43 PM EST by HHenderson]
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!
Link Posted: 5/12/2020 6:25:15 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By pezboytate:


Just send them a link to this thread!
View Quote

Exactly!
Link Posted: 5/12/2020 6:32:12 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By HHenderso­n:
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.

https://danieldefense.com/media/catalog/product/cache/9ecb5588c71cced9cf913f5b3a6f626c/m/f/mfr_10.0_updated_web_image_1.jpg

And great job! 
View Quote


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

Thanks!
Link Posted: 5/12/2020 10:29:32 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By pezboytat­e:


No problem.
View Quote


great post sir!
Link Posted: 5/12/2020 10:41:32 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By officerX:
Great post, thanks!  I've needed this in the past and will need it in the future!
View Quote

Link Posted: 5/13/2020 9:25:29 AM EST
Originally Posted By DCC83501:


great post sir!
View Quote

Originally Posted By Jack_T:

View Quote


Thanks!
Link Posted: 5/13/2020 10:03:51 AM EST
[Last Edit: 5/13/2020 10:04:40 AM EST by d16man]
Link Posted: 5/13/2020 10:57:29 AM EST
[Last Edit: 5/13/2020 10:58:53 AM EST by mmmboats]
Originally Posted By pezboytate:
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 fit any reason for being the length they are. Other rails were made to meet specific requirement. 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:

https://www.AR15.Com/media/mediaFiles/138579/BARREL_LENGTH_BASICS_jpg-1412816.JPG

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.

https://www.AR15.Com/media/mediaFiles/138579/CARBINE_FSB_jpg-1412868.JPG

https://www.AR15.Com/media/mediaFiles/138579/MID_FSB_jpg-1412835.JPG

https://www.AR15.Com/media/mediaFiles/138579/RIFLE_FSB_jpg-1412836.JPG

https://www.AR15.Com/media/mediaFiles/138579/CARBINE_LPGB_jpg-1412847.JPG

https://www.AR15.Com/media/mediaFiles/138579/MID_LPGB_jpg-1412848.JPG

https://www.AR15.Com/media/mediaFiles/138579/RIFLE_LPGB_jpg-1412849.JPG

https://www.AR15.Com/media/mediaFiles/138579/10_0_jpg-1412850.JPG

https://www.AR15.Com/media/mediaFiles/138579/10_4_jpg-1412851.JPG

https://www.AR15.Com/media/mediaFiles/138579/10_5_jpg-1412852.JPG

https://www.AR15.Com/media/mediaFiles/138579/11_6_jpg-1412853.JPG

https://www.AR15.Com/media/mediaFiles/138579/12_6_jpg-1412854.JPG

https://www.AR15.Com/media/mediaFiles/138579/13_9_jpg-1412856.JPG

https://www.AR15.Com/media/mediaFiles/138579/14_6_jpg-1412858.JPG

https://www.AR15.Com/media/mediaFiles/138579/14_9_jpg-1412859.JPG

https://www.AR15.Com/media/mediaFiles/138579/16_1_jpg-1412860.JPG

https://www.AR15.Com/media/mediaFiles/138579/16_3_jpg-1412861.JPG

https://www.AR15.Com/media/mediaFiles/138579/18_0_jpg-1412862.JPG

https://www.AR15.Com/media/mediaFiles/138579/20_0_jpg-1412863.JPG





Tacked and bolded your subject line.  ~d16man
View Quote



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.
Link Posted: 5/13/2020 11:13:29 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By d16man:
This is sticky worthy.  Great write up OP!

edit:  tacked, and I'm going to bold your title fo­r you.
View Quote


Cool. Hopefully people will reference it before posting a thread.
Link Posted: 5/13/2020 11:29:07 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By mmmboats:
 
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.
View Quote


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

Thanks!
Link Posted: 5/13/2020 11:43:15 AM EST
Very informative thanks for the post.
Link Posted: 5/13/2020 11:48:21 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By killerapp­lesauce:
Very informative thanks for the po­st.
View Quote


Thanks!
Link Posted: 5/13/2020 2:51:21 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By pezboytat­e:


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

Thanks!
View Quote


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!
Link Posted: 5/14/2020 7:58:02 AM EST
Fantastic post and love the illustrations! I occasionally still need something like this to reference.
Link Posted: 5/14/2020 8:11:27 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Outrider:
Fantastic post and love the illust­rations! I occasionally still need something ­like this to reference.
View Quote


Thanks!
Link Posted: 5/14/2020 8:20:48 AM EST
Link Posted: 5/25/2020 10:20:31 PM EST
This is pretty damn handy.
Link Posted: 6/15/2020 6:07:09 PM EST
[Last Edit: 6/20/2020 9:01:46 PM EST by Winn]
Very nice
Link Posted: 6/29/2020 10:52:51 AM EST
Got a drawing for a 12" barrel?
Link Posted: 6/29/2020 11:49:43 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Shqype:
Got a drawing for a 12" barrel?
View Quote


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"
Link Posted: 8/1/2020 10:35:53 AM EST
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)
Link Posted: 8/1/2020 10:58:01 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Brahmzy:
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)
View Quote
Concur!

And thank you for this thread, it is incredibly timely.
Link Posted: 8/1/2020 5:47:32 PM EST
Link Posted: 8/1/2020 5:56:23 PM EST
Originally Posted By Brahmzy:
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)
View Quote

Originally Posted By 96Ag:
Concur!

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

Originally Posted By bfoosh06:
Simply Awesome thread.

Thank You Very Much !!
View Quote


You're welcome.

I'll look into the pistol barrels but I have no idea on what actual length (ATF measured) they are.
Link Posted: 9/4/2020 1:47:19 PM EST
BTT
Link Posted: 9/4/2020 4:08:42 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By pezboytate:
BTT
View Quote

It's a sticky!  
Link Posted: 10/8/2020 8:14:06 PM EST
Just noticed this thread. Excellent post, OP

Link Posted: 11/26/2020 10:53:11 AM EST
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?
Link Posted: 11/26/2020 11:23:03 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Mhan7:
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?
View Quote


There's no industry standard for intermediate but we can definitely figure it out. What brand are you looking at?
Link Posted: 11/26/2020 12:12:37 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By pezboytate:


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


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).
Link Posted: 11/26/2020 12:20:59 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Mhan7:


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).
View Quote View All Quotes
View All Quotes
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Mhan7:
Originally Posted By pezboytate:


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.
Link Posted: 1/27/2021 3:26:30 PM EST
Originally Posted By pezboytate:

https://www.AR15.Com/media/mediaFiles/138579/20_0_jpg-1412863.JPG

Tacked and bolded your subject line.  ~d16man
View Quote


Does anyone know if this size of quad rail is even made or a custom shop that could make it?
Link Posted: 1/27/2021 3:33:19 PM EST
[Last Edit: 1/27/2021 3:37:03 PM EST by pezboytate]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Hybris:


Does anyone know if this size of quad rail is even made or a custom shop that could make it?
View Quote View All Quotes
View All Quotes
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Hybris:
Originally Posted By pezboytate:

https://www.AR15.Com/media/mediaFiles/138579/20_0_jpg-1412863.JPG

Tacked and bolded your subject line.  ~d16man


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.
Link Posted: 1/27/2021 4:33:27 PM EST
[Last Edit: 1/27/2021 4:34:25 PM EST by Hybris]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By pezboytate:


Quad? Not that I know of.
View Quote


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
Link Posted: 1/27/2021 6:32:34 PM EST
Link Posted: 1/28/2021 9:00:31 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By d16man:
That monstrum rail is either a) made by YHM, or B) a Chinese knockoff of a YHM rail.  Even down to the end cap it appears.
View Quote


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.
Link Posted: 2/12/2021 3:36:29 PM EST
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.

Attachment Attached File


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.
Link Posted: 2/14/2021 4:08:14 PM EST
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?
Link Posted: 2/14/2021 5:14:30 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By WillNY555:
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?
View Quote


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.
Link Posted: 2/14/2021 5:41:44 PM EST
[Last Edit: 2/14/2021 5:42:41 PM EST by WillNY555]
Originally Posted By pezboytate:


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.
View Quote



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.
Link Posted: 2/14/2021 5:45:46 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By WillNY555:



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.
View Quote


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

Someone might trade you their 13" for it.
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