help with new DPMS .308 build issues

[James95]

Hi, before I found this forum, I built an AR308 (DPMS) rifle.  I knew there was a difference between the AR10 and AR308, so I stuck to a pretty much complete Aero Precision build, but I've definitely learned a ton since reading some info on this forum!  So thank you all for the great info!

Here's my problem:  I took it to the range, and it doesn't cycle properly.  I was using Norma 150 grain fmj ammo.  It was discharging the brass at 3oclock, but it never locked back on an empty mag and either was closed on an empty chamber (most of the time) or would catch the next bullet halfway and get it stuck against the lower receiver.

Here's my build:

Aero 18" 308 CMV rifle length barrel (bought as a complete upper from Aero)

Aero nitride 308 BCG and charging handle

Magpul gen 3 PMags

Here's the part I think is causing the issues: 

I bought the Aero 308 carbine buffer system: (standard Ar15 tube with the 2.5" 308 buffer and matching spring)

I reached out to Aero, and they said they tune the rifle length gas system to their rifle length buffer tube system and vice versa.  AKA I can't use the carbine buffer system with the rifle gas system.  They recommended buying the rifle length buffer system.  I'm planning on putting the PRS lite (if it ever comes out haha) on this build, so I'd really prefer to stick to the carbine buffer system.  I know every build is a bit different, but I was surprised considering others have done exactly my build with no issues.  Has anyone else had the same issues I have?

 

Now after reading this forum a bit, I've read that the 2.5" 3.8oz buffer is crap for the 308, and I'm not understanding why? 

Either way, I saw my options as these and really didn't like either for the reasons above:

-Swap to rifle buffer sytem

-drill out gas port more

Is there an option 3 where I can use a carbine buffer system that will properly cycle a rifle length gas system?

I.e. can I use the Armalite carbine buffer system with my build?

 

Thanks for the help!!

 

[Armed Eye Doc]

The easy choice is the Armalite carbine buffer kit.  It is a slightly longer tube but it will work fine with your Aero system.  It will guarantee your recoil system is correct.  Otherwise, you can get a 2.5 inch buffer that weighs closer to 5.4oz from KAK or heavybuffers.com.

You may still need to drill your gas port after eliminating the recoil system.

 

[jtallen83]

1 hour ago, James95 said:

Is there an option 3 where I can use a carbine buffer system that will properly cycle a rifle length gas system?

I.e. can I use the Armalite carbine buffer system with my build?

Answered your own question there, you have been reading 

That Aramalite carbine buffer system is what you want, I think you're the second one to get this same poor advice from Aero. Sometimes if you call Aramalite they can ship you a kit;

https://www.armalite.com/SACItem.aspx?Item=AR10REKIT01&ReturnURL=/Armalite/Product-Category/AR10-Parts-Accessories/Lower-Receiver-Parts&Category=ac614400-ff09-4cdf-9d35-419a654e7201

 

[James95]

Thanks!  Is there a noticeable difference between doing an armalite carbine buffer system and doing the 5.4oz 2.5” buffer with AR15 tube or are they pretty similar?

also, what is it that makes these superior to the carbine 3.8oz buffer that came with Aero’s setup? 

[98Z5V]

3 hours ago, James95 said:

Here's the part I think is causing the issues: 

I bought the Aero 308 carbine buffer system: (standard Ar15 tube with the 2.5" 308 buffer and matching spring)

That's the primary problem, right there.  The extension is hit or miss, unless they've fixed it - I have one from them that was an internal depth of 7.100" - and that's not right.

3 hours ago, James95 said:

I reached out to Aero, and they said they tune the rifle length gas system to their rifle length buffer tube system and vice versa.  AKA I can't use the carbine buffer system with the rifle gas system.  They recommended buying the rifle length buffer system. 

This is pure bullshiit.  You don't have to have a specific Rifle-and-Rifle combo to make a gun run.  However, the recoil system AND the gas system need to be balanced, to work properly. 

 

3 hours ago, James95 said:

Now after reading this forum a bit, I've read that the 2.5" 3.8oz buffer is crap for the 308, and I'm not understanding why?

Because the gun was designed to run on a 5.4oz buffer, in order to control the Ass and the Mass of the 308AR BCG and the cycling of a .308 Winchester round.  That's why.

Read this:

 

[James95]

Thanks for the link, ton of good info.  I think I’m following.  So to clarify, any 308 ar needs at least a 5.4oz buffer to reliably cycle different types of ammo and by using the lighter buffer, it has a small window of pressures/ammo it can handle in order to properly cycle since it cycles faster?

-my gas tube looks centered

-my buffer tube looks to be 7” but is a tad shy of being flush with the upper receiver

Do you think there would be a big difference between using the armalite carbine system vs the 2.5” 5.4 oz buffer with a springco orange spring?

Edited December 16, 2021 by James95

[98Z5V]

19 minutes ago, James95 said:

Thanks for the link, ton of good info.  I think I’m following.  So to clarify, any 308 ar needs at least a 5.4oz buffer to reliably cycle different types of ammo and by using the lighter buffer, it has a small window of pressures/ammo it can handle in order to properly cycle since it cycles faster?

-my gas tube looks centered

-my buffer tube looks to be 7” but is a tad shy of being flush with the upper receiver

Do you think there would be a big difference between using the armalite carbine system vs the 2.5” 5.4 oz buffer with a springco orange spring?

You won't find a 2.500" buffer that you can get to 5.4oz, ever.  The max you can get that thing is 5.3oz, with two tungsten weights, inside a stainless steel body.  But that works.  The Sprinco Orange spring was specifically designed for a 2.500" 5.4oz buffer, running inside a receiver extension that's only 7.000" internal depth.  They made that spring, for that recoil system, for a .308AR. 

You can run that setup, and it'll work.  I run it in my .260 Rem AR, just to prove that it works, and that gun RUNS...  I did that for THIS reason.  That's the ONLY .308AR that I have with that recoil system.  Every other one runs the Armalite AR-10 Carbine Recoil System.  My .260 is the one, though.  It's the goods. 

So, next for you, is verifying your gas port diameter.  Your gas tube position is perfect.  Verify your gas port diameter.  And fix your recoil system.  You do that, and make sure your gas port diameter is right, and your gun will run anything you throw at it. 

Edited December 16, 2021 by 98Z5V

[98Z5V]

On the 18" .308AR barrel with rifle gas and a 0.750" gas block journal diameter, your gas port diameter HAS to be 0.093"~0.096" sized.  Or, it doesn't work. 

Check that first, and fix that first.

[James95]

Thank you for the great info.  I bought the springco orange spring and 5.3 oz buffer from KAK, Expo arms, and AR stoner (for comparison, will return the ones I don’t use), haven’t gotten the AR one yet but the other two both weight 5.1-5.15oz.  Is this an issue? Also have you heard any issues with any of the 3 products? Doesn’t seem it matters a ton as long as weight and size are to specs for the buffer?  I also noticed the light buffers are underweighted by .1oz.

On that note, since the bcg is the vast majority of the reciprocating mass, shouldn’t I be taking that into account when figuring out what buffer and spring I need?  Or are all standard bcgs the same weight?  I couldn’t find Armalites bcg weight..

My gas port is  as Aero says, 0.089”.  I bought drill bits ranging from 0.093-0.1” and will hopefully get to drilling it out soon. Is it possible to drill it to a size where it will reliably cycle pretty much any ammo, or do I need to drill bigger and put on an adjustable gas block to do it?  If so, do I just start at a 0.093” hole and test it to work my way up?

Edited December 18, 2021 by James95

[DustBuster]

My gas port is .093 and I have a KaK Heavy buffer and their spring and have never had one problem with cycling.  Carbine length stock and rifle length port.  My el-cheapo adjustable gas block has been screwed with twice but never seemed to do anything noticeable...I probably don’t need it.  Although at the moment I’m not sure if it’s nearly closed or nearly full open or somewhere in the middle... For a barrel test I’m going to do I will be attempting to shut her completely off and fire without my gas tube:  So Maybe adjustable gas block is still sort of advisable 

Edited December 18, 2021 by DustBuster

[98Z5V]

6 hours ago, James95 said:

haven’t gotten the AR one yet but the other two both weight 5.1-5.15oz.  Is this an issue? A

That won't be an issue.  

[James95]

Sorry for the very delayed update, but I finally got around to drilling and testing.  I drilled my gas port to 0.0945" and with the 5.3oz buffer/springco orange spring it cycled cheap 150grain Tula ammo beautifully, 100 rounds no malfunctions.  It was throwing the steel 4-5 oclock, so I decided to drill out to 0.096", and it threw the Tula 3-4 oclock but with surprisingly MUCH greater recoil.  I'm going to be using a suppressor so plan to put an adjustable gas block on anyway, but I'm helping some family with identical builds and am recommending 0.0945" for their 18" aero uppers.  

Anyone else experience such difference on recoil with such a small change in gas port diameter?

Anyways, thanks again all for the advice and help!  Ain't nothing quite like a 308 ar build!

[shooterrex]

You gave it enough gas to have the buffer bottoming out in the buffer tube. That's where the increased recoil came from.

[Harveyshot]

I just must have been lucky on my one and only LR-308 build. It's an 80% polymer lower and I bought parts from a bunch of different sellers and except for the bolt catch all eventually worked out in the end with no drilling or modifications to any of the parts, just some tweaking. The buffer assembly I bought (Luth-AR LR-308/AR-15 MBA-2 Skullaton Stock Assembly A2 Tube .308 Rifle Buffer And Spring Dark Earth MBA-2FK308 | Cheaper Than Dirt) didn't specify the buffer weight only that it was 'standard' but it worked. I did buy an adjustable gas block made by Sadlak Ind. and that, I think, made all the difference. Once I opened it up, all the FTLs and FTEs went away. The Armalite AR10A bolt catch replaced the one in the LPK. It has a 20" barrel from CDC and rifle length gas.

I would have been up poop creek if mine had required drilling.

Yes, the red dot optic color doesn't match the rifle but that's what happens when you order from the picture without reading the description. It 'looked' FDE. (sigh)

Hope your 308 works out well, James.

[98Z5V]

8 hours ago, shooterrex said:

You gave it enough gas to have the buffer bottoming out in the buffer tube. That's where the increased recoil came from.

^^^   That's exactly what happened, there.  Which is what it's supposed to do.