.243 AR Gas system

[Jk2paintworx]

I am starting on a lightweight .243 that I will be using for coyote hunting. I have built several AR15 platforms but this is my first large frame. I am using a V7 rec set and a 18” X Caliber .243 1:10 twist barrel with Rifle length gas port and a adjustable gas block.

But I’m not sure what BCG/Spring system to use. I have my eye on the JP LMOS because It’s only 9oz but not sure the light carrier is enough weight to run the rifle properly. Also not sure what buffer/spring system to use.

Does anyone have experience with getting the 6mm AR10s to run well? They sound like they are a little more finicky than the AR15s And the parts are spendy when shaving weight so I wanna get it right the first time. I will be shooting suppressed 100% of the time and using lighter weight bullets 55-90grain.

thanks.

 

[DRAM40A1]

I have a 20” Bartlein +1 6 Creedmoor, a 22” JP rifle length gas 6 Creedmoor, and a 22” Seekins 243 with standard rifle gas and run them all on the same lower. I use JP FMOS and an Armalite 7 3/4 buffer tube with rifle spring and H3 carbine buffer. I’ve run a LMOS carrier with lighter projectiles but it seemed to open the bolt too early so I went back to full mass.

 

[wompa]

Seems like most high pressure cartridges went +1 or +2 gas, but with an adjustable block you should be able to tune the dwell time. I have a 6 creed on rifle gas w/heavy buffer. I run suppressed and can tune adequately to eject at 3 o’clock without extractor swipes. I think I’m open 3 or 4 clicks is all.

 

[larryh128]

I am starting on a lightweight .243 that I will be using for coyote hunting. I have built several AR15 platforms but this is my first large frame. I am using a V7 rec set and a 18” X Caliber .243 1:10 twist barrel with Rifle length gas port and a adjustable gas block.

But I’m not sure what BCG/Spring system to use. I have my eye on the JP LMOS because It’s only 9oz but not sure the light carrier is enough weight to run the rifle properly. Also not sure what buffer/spring system to use.

Does anyone have experience with getting the 6mm AR10s to run well? They sound like they are a little more finicky than the AR15s And the parts are spendy when shaving weight so I wanna get it right the first time. I will be shooting suppressed 100% of the time and using lighter weight bullets 55-90grain.

thanks.

From playing with AR-10's in 6.5 Creed, my opinion is that you need a BCG with some mass. Also either an adjustable gas block or gas key, I use a Tubb flat wire spring and a 9 oz buffer. You need to keep it locked up as long as possible. I don't believe a light BCG is what you want.

 

[Luke G]

With that gas system it will be tough to get it to run correctly with a low mass BCG. I have had success using a low mass carrier combined with adjustable gas block and higher power buffer spring. This was on a 20” 6mm creed with a +1 gas system so a bit different but close. If cutting overall weight is your goal then I would go for it but, if weight isn’t super important I would take the easier route and go full mass.

 

[Yondering]

With that gas system it will be tough to get it to run correctly with a low mass BCG. I have had success using a low mass carrier combined with adjustable gas block and higher power buffer spring. This was on a 20” 6mm creed with a +1 gas system so a bit different but close. If cutting overall weight is your goal then I would go for it but, if weight isn’t super important I would take the easier route and go full mass.

I can't agree with your reasoning there. Don't look at the BCG as a place to save overall weight on the rifle; instead look at it from the perspective of reducing the reciprocating mass.

Personally I've had very good results with light weight bolt carriers in the large frame AR system, but they can be more picky about gas adjustment and using different loads. In my experience - if you want the rifle to be reliable with a wide range of loads, basically running whatever ammo you put in it, then use a full mass bolt carrier and H buffer, and tune the gas for reliability with the weakest load you'd use.

On the other hand, if you want to use one particular load, and want to optimize the rifle to shoot as softly as possible with the least movement from BCG movement, then a low mass carrier can be a really good choice. Just make sure to tune the gas for that load.


Also just a general comment since I've seen some do this - it makes zero sense to use a heavy buffer with a low mass carrier. The buffer and carrier mass work together, and if you use a heavy buffer with that light weight carrier, you might as well just use a heavy carrier instead. With my low mass carriers I typically use a C buffer, or sometimes even lighter (replacing some of the steel weights with aluminum); buffer mass can and should be matched to the carrier.

 

[Luke G]

I can't agree with your reasoning there. Don't look at the BCG as a place to save overall weight on the rifle; instead look at it from the perspective of reducing the reciprocating mass.

Personally I've had very good results with light weight bolt carriers in the large frame AR system, but they can be more picky about gas adjustment and using different loads. In my experience - if you want the rifle to be reliable with a wide range of loads, basically running whatever ammo you put in it, then use a full mass bolt carrier and H buffer, and tune the gas for reliability with the weakest load you'd use.

On the other hand, if you want to use one particular load, and want to optimize the rifle to shoot as softly as possible with the least movement from BCG movement, then a low mass carrier can be a really good choice. Just make sure to tune the gas for that load.


Also just a general comment since I've seen some do this - it makes zero sense to use a heavy buffer with a low mass carrier. The buffer and carrier mass work together, and if you use a heavy buffer with that light weight carrier, you might as well just use a heavy carrier instead. With my low mass carriers I typically use a C buffer, or sometimes even lighter (replacing some of the steel weights with aluminum); buffer mass can and should be matched to the carrier.

Just depends on what your goals are with the build. The OP mentioned he is doing a lightweight build but wants reliability so my comment was directed at those aspects. Yes the lower weight system reduces reciprocating mass for lower recoil which is usually the primary goal and isn’t to be ignored but, he was asking about weight and reliability. A low mass system in an lr308 build can reduced overall weight of the rifle by about half a pound which is significant weight savings in a hunting rifle. They do tend to be a little harder to tune so not knowing the OP’s experience level full mass carriers are the easy button.

With that being said I love low mass carriers and run them in all but 2 of my large frame and small frame builds. I am currently working on building a large frame in 22 Creedmoor for coyote hunting that will also run one. If you don’t mind playing around with it I definitely recommend going the low mass route. But as Yondering mentioned they are definitely more finicky when it comes to switching loads around. He also mentioned disassembling the buffers and replacing the weights with aluminum which is another great way to reduced overall/reciprocating mass and cheaper than buying a lightweight buffer. I would look into using a higher pressure buffer spring as that can help add some bolt load and make it easier to tune and run.

 

[bfoosh006]

What stock are you going to use ? Carbine ? Rifle ?

IMHO... there are Pros and Cons to both a "308" LtWt BCG v. a "Normal" "308" weight BCG.

A LtWt BCG ( and buffer weight ) can alter perceived recoil from the less reciprocating mass ... however, IMHO, they also tend to unlock the bolt earlier, at a higher chamber pressure... and that can lead to extraction issues.
And higher cyclic speeds of the BCG.. which can lead to failures to ( combined with the extraction issues ) eject the spent cases.. which can turn into a overfeed of the next cartridge... feeding over the unejected case.

A standard BCG and a heavier buffer system leaves the Bolt locked milli-seconds longer. And that help extraction at a lower chamber pressure and leaves the primer better supported slightly longer as well... less ejector smear, and much easier on the extractor.

Adding an Adj. GB can help "ease" the momentum of the BCG ( vigorous cyclic speed ) and I use an Adj. GB on all my Large Frames.
The less vigorous cyclic speed also alters the perceived recoil.

All my Large Frame AR's ( 308's and 6.5CM's ) use .... full mass BCG's, heavy buffer weights, Tubbs 308 Flatwire recoil springs, SLR Adj. GB's, and Lantac brakes... regardless of the gas system length and barrel length. I have had no issues function wise.

They have all been reliable ( 9 of them ) ... with the heavy setups... and the Adj. GB makes a big difference in perceived recoil.

The various barrel manufacturers are Criterion, Ballistic Advantage, Faxon, FN CHF CL, PSA / DC Machine, Bushmaster... and Krieger.

Only the Krieger had a smallish gas port , IMHO appropriate sized gas port. ( Note... NO +1 / +2 gas system lengths. )


Bottom line to all of this is your gas port size. ... To big a gas port, vigorious cyclic speed and all the problems with that.

lots of people have had success with LtWt BCG's and Adj. GB's... I just really like the heavy way I have mine setup. Very soft and smooth firing.

 

[flyright]

I would have never chosen a low mass bolt/carrier until JP recommended the LMOS for my LRP-07 6.5CM when I was ordering and talking with Ben. Between that and the AGB it is the softest shooting 6.5 AR that I've ever felt - a true half moa rifle to boot.

 

[bfoosh006]

I would have never chosen a low mass bolt/carrier until JP recommended the LMOS for my LRP-07 6.5CM when I was ordering and talking with Ben. Between that and the AGB it is the softest shooting 6.5 AR that I've ever felt - a true half moa rifle to boot.

Can you share some specifics about the build ? I am curious ... buffer weight, barrel length and gas system length, recoil spring, muzzle brake, brand of Adj GB / how many clicks open ?
And if you happen to know the gas port size.

Again just curious... ironically, everyone that has fired mine, says they are surprisingly very soft shooting. ( Again everything heavy 9.3oz 308 rifle buffer heavy )

Maybe the Adj. GB does more to calm the felt recoil then I thought.

 

[Yondering]

Maybe the Adj. GB does more to calm the felt recoil then I thought.

Yes, of course. Correctly tuning the gas system is vital on a large frame AR, and makes a big difference in felt recoil.

And to the comments above about lightweight carriers unlocking early - sure, if you don't adjust the gas system to match the light weight carrier. Gas adjustment is mandatory, not just recommended when you reduce the reciprocating mass. When it's done correctly though, the rifle can run just as well as a heavy carrier setup, but with less recoil. And that is less recoil in both directions, bolt opening and bolt closing, not just one direction like a bolt action.

 

[Thermshot49]

I'm in this same position. Building an ultra light 243. Im hung up on bcg and buffer setup. Im only going to run 58 to 75 grain bullets suppressed.

 

[iktomi]

BLUF: If you are looking for reliable extraction with higher pressure loads, I'd lean towards higher weight systems.

--

I've had the chance to build a couple 6.5 Creedmoors with light-weight being a focus.

Initially, I simply "scaled up" some lightweight AR-15 builds to AR-10 components.

TI bolt carriers, light buffers, Supurlative gas blocks. Piles of springs.

I have the most time behind a 22" proof +2 gas, running a TB Ultra 7. Started with a JP HP bolt, V7 TI carrier. (ASIDE: both components were very nicely machined parts.)

(The suppressed gun was far more challenging to tune then the unsuppresssed gun.)

To keep the TI BC closed long enough, I ran an H3 carbine buffer with a JP extra power 308 spring.

Later, I got it running with the stiffest spring in a JP Captured Spring with two tungsten weights... eventally. I still kept a close eye on the chamber for fouling I associate with early extraction.

Still had some occasional swipe marks pushing Berger 130 T Hybrids around 2900 that weren't there in load development.

Rethought what I was seeing, and have started playing with the JP Variable mass carrier with two tungsten weights... and the corresponding pile of springs and gas settings.

Running heavy has reduced (I'd say "eliminated" but sample size is low) pressure indicators (consistent with intial load devlopment), and my current set up has extreemly restricted gas with a lower spring rate.

The down side is obvious: adding mass to the action adds mass to the gun. That wasn't the intial goal.

But fighting pressure and extraction wasn't the goal either.

Conclusion: If you are looking for a less finiky build with reliable extraction with higher pressure loads, I'd lean towards higher weight systems in your BCG and buffers for your starting point.

 

[Thermshot49]

iktomi, thanks for all that info. As much as I've gone out of my way to get the lightest parts available I think I will just get the full mass bcg at this time. I don't think I have the experience to tinker with the titanium or low mass stuff. Maybe down the road when I'm ready to shed a little more weight off the final project I will get into the light bcg game. Thanks again.