12.5” 308 SBRs and related

[GorillaTactics2089]

So, I’m looking at doing a .308 12.5-13” SBR. Just wondering who all has done this and what parts were used as far as gas tube length, buffers, and what luck y’all are having with what ammo and such. Mine will be suppressed with a HuxWrx flow through. I’ll end up putting hand loads in it. Just getting a feel for y’all’s most reliable setups.

[shepp]

@Rsquared

[shooterrex]

Once upon a time Sig sold a bunch of 12" 308 barrels to CDNN Sports. Several from this forum built pistols/sbr's from these barrels. They had a really weird gas port location. White Oak Armory makes custom length gas tubes. I built 2 pistols. Both run and shoot fine. Not suppressed though. Faxon also makes those length barrels if I remember correctly. 

Armalite 308 carbine buffer tube and spring w/a h3 buffer.

Both pistols run 147-175gr ammo just fine.

Edited August 25 by shooterrex

[98Z5V]

^^^  I have one of those, and kind of pioneered the gas tube length issue - how to measure, and get what you need.  Those barrels were released with two different gas port locations.

So, I have that 12.5" barreled .308 Win gun, and a 13.5" barreled .308 Win gun.

OP, if you run into issues building one of these, post up here.  Both mine run like a pissed off Banshee...

Edited August 26 by 98Z5V

[GorillaTactics2089]

3 minutes ago, 98Z5V said:

^^^  I have one of those, and kind of pioneered the gas tube length issue - how to measure, and get what you need.  Those barrels were released with two different gas port locations.

So, I have that 12.5" barreled .308 Win gun, and a 13.5" barreled .308 Win gun.

OP, if you run into issues building one of these, post up here.  Both mine run like a pissed off Banshee...

Absolutely brother. You running fixed or adjustable gas blocks? Every 10 I own has superlative arms adjustables.

[98Z5V]

Non-adjustable.  I believe in balancing the gas system to the recoil system.  Get the gun right first, before you introduce suppressors.  After suppressor introduction, if you need to change to an adjustable gas block - that's when you do it.  Not out of the gate.  Make the gun right, first.  

[GorillaTactics2089]

2 minutes ago, 98Z5V said:

Non-adjustable.  I believe in balancing the gas system to the recoil system.  Get the gun right first, before you introduce suppressors.  After suppressor introduction, if you need to change to an adjustable gas block - that's when you do it.  Not out of the gate.  Make the gun right, first.  

I like that. To be fair, I’m planning on running a flow through so it should run quite well on a non adjustable.

[98Z5V]

1 hour ago, GorillaTactics2089 said:

I like that. To be fair, I’m planning on running a flow through so it should run quite well on a non adjustable.

Get the Gas System and Recoil System balanced - and it should do great with a flow-through, and non-adjust block.   Suppressed and unsuppressed. 

Tell me what your barrel config idea is, and I can tell you what the gas port diameter should be. For a 12.5, you should run a Carbine Gas System.  If you decide to run a 13.5, you can get away with a midlength gas system.  I can give you gas port diameters for both configs. 

12.5 and midlength gas is tough to pull off, even in a 5.56 gun.  Trust me.  I have a few of those.  Dwell time becomes the biggest issue.

Edited August 26 by 98Z5V

[MtnMike]

On the topic of 308 SBRs, any thoughts on whether I could just swap this 10" upper out https://www.midwayusa.com/product/1028296351?pid=518598 with the current one on an out of production Windham Weaponry AR 308 with a 16" barrel. 

Love the old WW, it's been incredibly reliable and is quite accurate.  However, it's also a bit redundant now since I obtained a Daniel Defense DDV-V3.  I've already made a few mods to the WW to further lighten it.  Thinking a 10" barrel would make for a solid package for hunting in an area with a lot of undergrowth and generally short sight lines.

[GorillaTactics2089]

10 hours ago, 98Z5V said:

Get the Gas System and Recoil System balanced - and it should do great with a flow-through, and non-adjust block.   Suppressed and unsuppressed. 

Tell me what your barrel config idea is, and I can tell you what the gas port diameter should be. For a 12.5, you should run a Carbine Gas System.  If you decide to run a 13.5, you can get away with a midlength gas system.  I can give you gas port diameters for both configs. 

12.5 and midlength gas is tough to pull off, even in a 5.56 gun.  Trust me.  I have a few of those.  Dwell time becomes the biggest issue.

I’m doing a 12.5” carbine length. My goal is for this to run well suppressed and unsuppressed, be reliable, and not beat the crap out of itself. It’s gonna stay in the experimental state until I feel like it’s proven it’s to be reliable for carry. I run the piss out of all my runs so if it can’t keep up, back to the safe till I can get it right. 😂

[98Z5V]

17 hours ago, GorillaTactics2089 said:

I’m doing a 12.5” carbine length. My goal is for this to run well suppressed and unsuppressed, be reliable, and not beat the crap out of itself. It’s gonna stay in the experimental state until I feel like it’s proven it’s to be reliable for carry. I run the piss out of all my runs so if it can’t keep up, back to the safe till I can get it right. 😂

I can tell you what you need for a recoil system - but I want to hear from you what your planned recoil system is first - and discuss that further, if necessary. Let it rip.  What are you planning for your recoil system, and how did that come to be your conclusion...  

[GorillaTactics2089]

On 8/27/2025 at 1:17 AM, 98Z5V said:

I can tell you what you need for a recoil system - but I want to hear from you what your planned recoil system is first - and discuss that further, if necessary. Let it rip.  What are you planning for your recoil system, and how did that come to be your conclusion...  

For this one I was going with a Wilson Combat AR10 carbine spring, Slashes’ extra heavy 6.5 oz buffer and a full mass carrier. 
 

I have used slashes buffers for most of my projects to slow down the bolt speed. Has been great in my suppressed setups. To be fair, my main rifles are rifle length systems, heavy buffers, with adjustable gas blocks, and tuned to suppressor use. They have all ran great unsuppressed, though.

 

My worry here is that a carbine length .308 SBR would have a janky dwell time without a heavy enough buffer. Reasons I’m here lol. I don’t really mess with SBRs and I like the additional knowledge.

[98Z5V]

8 hours ago, GorillaTactics2089 said:

For this one I was going with a Wilson Combat AR10 carbine spring, Slashes’ extra heavy 6.5 oz buffer and a full mass carrier. 
 

I have used slashes buffers for most of my projects to slow down the bolt speed. Has been great in my suppressed setups. To be fair, my main rifles are rifle length systems, heavy buffers, with adjustable gas blocks, and tuned to suppressor use. They have all ran great unsuppressed, though.

 

My worry here is that a carbine length .308 SBR would have a janky dwell time without a heavy enough buffer. Reasons I’m here lol. I don’t really mess with SBRs and I like the additional knowledge.

Well, part of the recoil system is the receiver extension, and you only mentioned the buffer weight and the spring.  Never mentioned what extension you planned, and that's a GIANT part of the Recoil System on a Large Frame AR.  Ditch that WC spring, right off the bat.  Forget about it.  WC doesn't always get their gas port diameters right, in their large frame barrels, and I have a pile of proof of that, because I run WC barrels on all my weird caliber Large Frames.  Buffer choice, in company?  Very good.  I run alot of Slash's buffers myself.  

What receiver extension are you planning?  I'm asking you this directly, because of the buffer you listed, and I already know the length of that buffer.  ,,,And, you have better options, that will work better, for this build.  

I'm running them in my 12.5" and 13.5" .308ARs...   

[GorillaTactics2089]

17 hours ago, 98Z5V said:

Well, part of the recoil system is the receiver extension, and you only mentioned the buffer weight and the spring.  Never mentioned what extension you planned, and that's a GIANT part of the Recoil System on a Large Frame AR.  Ditch that WC spring, right off the bat.  Forget about it.  WC doesn't always get their gas port diameters right, in their large frame barrels, and I have a pile of proof of that, because I run WC barrels on all my weird caliber Large Frames.  Buffer choice, in company?  Very good.  I run alot of Slash's buffers myself.  

What receiver extension are you planning?  I'm asking you this directly, because of the buffer you listed, and I already know the length of that buffer.  ,,,And, you have better options, that will work better, for this build.  

I'm running them in my 12.5" and 13.5" .308ARs...   

https://www.vsevenweaponsystems.com/lower-parts/heavy-steel-a5-buffer-tube

 

A5 type

[98Z5V]

Damn good thing I asked.  

That buffer you selected won't work with the receiver extension you've stated. You have the right internal depth on the receiver extension, for a large-frame ar - just get a different buffer from Slash.  

[98Z5V]

For a recoil spring, now that I know what you're intending - shitcan that WC offering, completely.  Once you pick the appropriate buffer, run one of two springs, only.  Armalite EA-1095 spring, or Sprinco Red spring. 

 

[98Z5V]

@GorillaTactics2089, I'll cut right to it.  If you run an A5 extension or the Armalite AR-10 Carbine Reciever extension, or the MagPul SR-25/M110 extension - those are all 7 5/8" internal depth, and they are the original design for the AR-10.  You must run a buffer that's 3.250" long.  The buffer you first listed is a great buffer, but it's 2.500" long, made for a Large Frame running an AR-15 Carbine Receiver extension, which is 7.000" internal depth.  True design depth is 6 15/16" internal depth. Every small frame out there does just great with 7.000" internal, though.

So, the buffer you listed is too short to run with the extension you've chosen.  The extension you've chosen is the proper depth needed to balance the recoil system to the gas system, on a Large Frame.  If you order this buffer from Slash, if comes with an Armalite EA-1095 recoil spring...  This is all you need to finish your recoil system...

https://heavybuffers.com/xhcar.html

[GorillaTactics2089]

1 hour ago, 98Z5V said:

@GorillaTactics2089, I'll cut right to it.  If you run an A5 extension or the Armalite AR-10 Carbine Reciever extension, or the MagPul SR-25/M110 extension - those are all 7 5/8" internal depth, and they are the original design for the AR-10.  You must run a buffer that's 3.250" long.  The buffer you first listed is a great buffer, but it's 2.500" long, made for a Large Frame running an AR-15 Carbine Receiver extension, which is 7.000" internal depth.  True design depth is 6 15/16" internal depth. Every small frame out there does just great with 7.000" internal, though.

So, the buffer you listed is too short to run with the extension you've chosen.  The extension you've chosen is the proper depth needed to balance the recoil system to the gas system, on a Large Frame.  If you order this buffer from Slash, if comes with an Armalite EA-1095 recoil spring...  This is all you need to finish your recoil system...

https://heavybuffers.com/xhcar.html

My dude 💪💪💪