AR 6.5CM buffer system question

[ragin_trajan19]

Hi all,

Here's the build to get that out of the way:

Aero Upper, Lower, handguard, and BCG (BCG weights 17.5 oz w/ HP bolt)
24" Wilson Combat super sniper barrel
Leapers UTG AR308 A2 Style Fixed Buttstock (came with buffer (5.2 oz), spring, and tube)
Superlative Arms AGB (rifle length system)

I have over 250 rds through it so far. Recently I started reloading my own ammo in the last 6 months, and have a sub-MOA load developed. Ultimately, I don't like the stock and I want to upgrade to the PRS. The Leapers buffer system origionally was, and still is very stiff. Bolt would not lock open with the gas block fully open. I shot about 50 rds allowing for an initial break-in, but nothing changed so I cut small portions of the spring in increments until I achieved reliable cycling and was able to adjust the gas block for the factory loads I was shooting at the time.

Overall, it treats brass okay. Not bad, not great. Tiny bit of primer cratering and flattening, but nothing excessive. I do get a little bit of bending at the rim where the extractor grabs the case. Anywhere from .001-.003". Worse with softer brass (Hornady). Sometimes there's ejector swipe, but sometimes there isn't. If there is it's also not excessive.

In the last 40 rounds or so I've had inconsistencies with the bolt locking back even with the AGB wide open as I was doing load dev and I like to keep it open for that. After reading this forum and others, I do believe that the buffer system can contribute to this issues and also the issues above in terms of brass treatment. I know in a gas gun I can't expect brass to come out beautiful, but I do believe there's room for improvement. I'd really like to limit the rim bending as it makes bullet seating inconsistent, and measuring true case length/headspace difficult. Since I'm changing the stock, I figured upgrading the buffer system should coincide. I'll also note that charging the rifle is very difficult from the spring tension. This is my first large frame AR, so maybe that's expected...

My request for advice is as follows:

What should I be looking for in terms of buffer size/weight and the buffer spring itself? I find myself confused in most threads because people start talking about AR-15 buffer systems and I still don't understand the similarities and/or differences between AR-15 and AR-10 buffer systems. My current buffer is 5.2 oz.

People recommend the JP SCS, but I'm trying to avoid the $140-200 price tag for a buffer. I'm wondering if there's a more middle of the ground alternative. I'm not looking for budget parts as my stock and buffer system already is so to speak. If I do need to shell out the green backs to achieve what I want, then so be it.

This is my first post here. Thanks in advance for any forthcoming response. It's all appreciated.

-RT

 

[bigjake83]

Welcome to the Hide.

You're running a 24" 6.5CM barrel with a rifle length gas system, thats your first problem and unfortunately theres nothing you can do about that.

The 6.5CM likes a shorter Dwell Time thats why any quality system will run a Rile Lengh +2 Gas system or even a +3 on a 22" -24" barrel which will also lower the chamber pressure resulting in a smoother/softer cycling and less abuse on your brass.

If trying to keep cost down I would pick up a Tubbs Flat wire AR308 spring and start there.

Next you're running a H3 buffer weight system which isn't a bad idea with the rifle gas system and if your Barrel was shorter I would say stick with that but since you're running a 24" barrel I'd bring that weight down to a H2 4.65-ish Oz system.

I know of a place you can buy (2) Rifle length Buffers for $20 one in H1 and the other H2, if interested send me a private message.

 

[msgriff]

Make sure that you have the correct gas tube length, it should end at the center of the cam pin cut-out in the upper. Verify that you can manually operate the bolt catch and that it's engaging the bolt, not the carrier. Check the distance between the bolt lugs and the bolt catch at full pull-back of the charging handle

 

[ragin_trajan19]

Welcome to the Hide.

You're running a 24" 6.5CM barrel with a rifle length gas system, thats your first problem and unfortunately theres nothing you can do about that.

The 6.5CM likes a shorter Dwell Time thats why any quality system will run a Rile Lengh +2 Gas system or even a +3 on a 22" -24" barrel which will also lower the chamber pressure resulting in a smoother/softer cycling and less abuse on your brass.

If trying to keep cost down I would pick up a Tubbs Flat wire AR308 spring and start there.

Next year you're running a H3 buffer weight system which isn't a bad idea with the rifle gas system and if your Barrel was shorter I would say stick with that but since you're running a 24" barrel I'd bring that weight down to a H2 4.65-ish Oz system.

I know of a place you can buy (2) Rifle length Buffers for $20 one in H1 and the other H2, if interested send me a private message.

Thanks for the response. Can you elaborate on your rationale for potentially going for a lighter buffer? From what I've pulled thus far, keeping the buffer heavy keeps the bolt locked a tiny bit longer allowing for pressure to drop. What's the relationship between the buffer weight and my setup?

-RT

 

[ragin_trajan19]

Make sure that you have the correct gas tube length, it should end at the center of the cam pin cut-out in the upper. Verify that you can manually operate the bolt catch and that it's engaging the bolt, not the carrier. Check the distance between the bolt lugs and the bolt catch at full pull-back of the charging handle

These are all verified. If I pull the charging handle all the way back, the catch will grab the bolt. It was only upon initially building before cutting the spring and firing with an open gas block that the bolt would not lock back, or if it would it was catching the carrier and not the bolt. After cutting the spring and tuning the gas block it had reliable cycling for over 100 rds. Just last weekend when shooting my own loads (doing a refined ladder increase) I started getting no lock back on the last round with an open gas block. Near max charge but not exceeding.

There a good 1/4" or so of gap between the bolt face and catch when the charging handle is pulled back fully.

-RT

 

[bigjake83]

Thanks for the response. Can you elaborate on your rationale for potentially going for a lighter buffer? From what I've pulled thus far, keeping the buffer heavy keeps the bolt locked a tiny bit longer allowing for pressure to drop. What's the relationship between the buffer weight and my setup?

-RT

You stated that you're having issues with the bolt locking to the rear, correct?

Then you modified the rifle length spring to get the bolt to lock to the rear which ended in ultimate failure because you're still having inconsistencies.

More Buffering weight isn't always the answer obviously your rifle isn't producing the proper amount of gas to reliably cycle your action so you either need less Mass or to open up your gas port.. comprende?

So starting over replace the buffer spring you fucked up with a quality spring like Tubs flat wire and lower your buffer weight to an H2.

 

[ragin_trajan19]

You stated that you're having issues with the bolt locking to the rear, correct?

Then you modified the rifle length spring to get the bolt to lock to the rear which ended in ultimate failure because you're still having inconsistencies.

More Buffering weight isn't always the answer obviously your rifle isn't producing the proper amount of gas to reliably cycle your action so you either need less Mass or to open up your gas port.. comprende?

So starting over replace the buffer spring you fucked up with a quality spring like Tubs flat wire and lower your buffer weight to an H2.

Makes sense... Thanks!

 

[Hardholder439]

This is like asking 10 doctors to come up with the same diagnosis.
I have a 26" Krieger barrel in mine... standard rifle length gas system. I went with a heavy buffer.

AR-STONER Heavy Buffer LR-308 Rifle

AR-STONER offers replacement components for the AR-15 and LR-308 These high quality components provide excellent fit and reliable function. AR-STONER...

www.midwayusa.com

Tubb flatwire spring.

Tubb Precision AR-10 Flatwire Buffer Spring (Fits both Carbine and Standard length stocks)

High Performance Tubb Precision Flatwire Stainless Steel Buffer Spring for AR-10 Rifles and Carbines

www.davidtubb.com

An adjustable clamp on SLR gas block -3-5 clicks out from full closed and the rifle has functioned perfectly since it was built.

My 20" .308 using a factory Aero built carbine lower wouldn't cycle the bolt completely with the gas system full open. It was also an SLR clamp on adustable gas block. I failed to get the gas port perfectly aligned. Two more attempts at alignment later, I got it right and it's 100% now. Obviously, if you are using a dimpled block, this would not be a possible cause.

 

[ragin_trajan19]

This is like asking 10 doctors to come up with the same diagnosis.
I have a 26" Krieger barrel in mine... standard rifle length gas system. I went with a heavy buffer.

AR-STONER Heavy Buffer LR-308 Rifle

AR-STONER offers replacement components for the AR-15 and LR-308 These high quality components provide excellent fit and reliable function. AR-STONER...

www.midwayusa.com

Tubb flatwire spring.

Tubb Precision AR-10 Flatwire Buffer Spring (Fits both Carbine and Standard length stocks)

High Performance Tubb Precision Flatwire Stainless Steel Buffer Spring for AR-10 Rifles and Carbines

www.davidtubb.com

An adjustable clamp on SLR gas block -3-5 clicks out from full closed and the rifle has functioned perfectly since it was built.

My 20" .308 using a factory Aero built carbine lower wouldn't cycle the bolt completely with the gas system full open. It was also an SLR clamp on adustable gas block. I failed to get the gas port perfectly aligned. Two more attempts at alignment later, I got it right and it's 100% now. Obviously, if you are using a dimpled block, this would not be a possible cause.

Good to know...

How do these rifles treat your brass?

I think I have my game plan now. I want to put a PRS-lite stock (current one is a POS, uncomfortable, and hard to get a good line of sight through my scope especially at high power) so I need to get a carbine buffer and tube anyways. I'm going to go for a Tubb flatwire, and carbine buffer (H2). Mess around with the weight if need be...

-RT

 

[msgriff]

Good to know...

How do these rifles treat your brass?

I think I have my game plan now. I want to put a PRS-lite stock (current one is a POS, uncomfortable, and hard to get a good line of sight through my scope especially at high power) so I need to get a carbine buffer and tube anyways. I'm going to go for a Tubb flatwire, and carbine buffer (H2). Mess around with the weight if need be...

-RT

As you are probably already aware, the PRS Lite can only use an AR-15 carbine or intermediate (A5) receiver extension.
When used on a large frame AR. the AR-15 carbine RE (~7" inside depth) requires a 308 short carbine buffer (~2.5" long). Those are commonly available in the mid-5oz range, and usually max out in weight at 6.5oz. Heavier than that would be a custom job. This route requires a 308 carbine spring, but there are other springs, like the Tubb, that also work.
If you use an intermediate length RE (~7 3/4" inside depth) the correct buffer is the AR-15 carbine buffer (~3.25"). Most use an H3 or H2. Those buffers max out at around 8.5oz, but there are a couple odd options or a custom. If you go with the intermediate RE and want to be able to adjust the buffer weight, there are a couple kits out there where you assemble your own buffer. This RE/buffer set-up uses an AR-10/308 rifle length spring with some exceptions, including the Tubb.
There are a few lesser known options for higher operating system mass.

 

[HALFMVSQUARED]

I may have a related problem. I built a DPMS Gen 1 LR308 lower and bought an Aero Precision M5 barreled upper receiver in 6.5 CM with a 24 inch barrel. I question if I have the right buffer, the right buffer spring, and if I may need a rifle length buffer tube instead of the shorter carbine tube.

While this lower receiver works fine with my other barreled upper in 308, it fails to fire about 30% of the time with the new AP 6.5 CM barreled upper. Not sure where to go from here. When it fails to fire, there is no mark on the primer as if the firing pin was blocked, maybe because the bolt didn't go all the way into battery? Do I need to add an adjustable gas block?

Suggestions? HELP!