LR243 Gas & Timing issues

[Bruce]

Hello everyone,

First, I have built two LR-308 carbine projects and two AR-47 carbine projects with no issues at all. But I have an LR-243 build that has stubborn cycling issues. Before we get into that, here are the build specifics as they stand today:

Polymer80 Warrhogg 308 Lower

DPMS LR-308 Lower Parts Kit

AR-15 Carbine Lower Receiver Extension (7” inside depth)

HeavyBuffers CAR-10 2.5”, 5.6oz Buffer

DPMS 308 Carbine Buffer Spring, P/N F1000588 10-7/8” OAL, 29 Coils, .073” Wire

AR Stoner 22” AR-10 .243 Win 1:10 Barrel w/.080” Rifle Length Gas Port

Superlative Arms .936 Adjustable Gas Block

AR-10 Rifle Length Gas Tube 15-1/2” OAL

Aero Precision DPMS 308 Upper

Brownell's LR-308 Bolt Carrier Group

Ammunition:

Federal 55 grain Varmint & Preditor

Symptoms:

With a non-adjustable gas block and 2.5”, 3.8 oz buffer the system is so severely over gassed and improperly timed that it tears the rims off of the brass and it has to be tapped out of the chamber with a cleaning rod. Adding the SA adjustable gas block opened 2 full turns (8 clicks) allows for extraction at 3:00 but no bolt hold open. Opening the gas an additional click (9 clicks) allows for extraction at 3:00 with no BHO but bends the case rim. An additional click open (10 clicks) and the rim is torn off the case again.

Things I have verified:

The bolt can be manually locked to the rear

The gas tube extends to middle of the pivot pin cutout in the upper and is centered

The gas block is aligned and has the usual carbon ring

Bolt head-space has been checked and verified

The gas key is tight, staked and in the correct location

Primers are slightly flattened with no ejector marks or bolt swipes

Buffer spring is not “stacking” in the extension

The bolt is well lubricated

The fired round count is 80

Troubleshooting I have done:

Swapped the buffer spring from one of my working .308 carbine builds with AR-15 7” extension with no change

Swapping the standard 3.8 oz buffer for the CAR-10 5.6 oz buffer allowed for decent extraction but no BHO

Things I am contemplating:

Is the gas port large enough at .080”?

Is the spring length and coil count proper for a 7” extension?

Some references say the proper 308 spring for 7” extension is 9-3/4” w/25 coils.

Should I change to a 7-3/4” 308 carbine extension?

Should I try a Sprinco Orange buffer spring? (supposed to be made for 308 w/7” extension)

Should I dig a hole, bury this barrel and forget this caliber?

Looking forward to your thoughts and comments.

[jtallen83]

Try a little chamber polishing. Flitz on a chamber mop and chucked in a drill. It will highlight any irregularities in the chamber as well.

[98Z5V]

@Bruce, welcome aboard.  Go rip up the intro section and tell us a little bit about yourself.  You have issues in the recoil system and the gas system, in this configuration. 

[mtrmn]

Tried a heavier bullet? Just a thought.

[Bruce]

4 minutes ago, mtrmn said:

Tried a heavier bullet? Just a thought.

Thanks, Yes, tried Nosler 95 grain partitions over H4895 and didn't really see any difference. But that was before I got the CAR-10 heavy buffer so..........

 

[Bruce]

22 hours ago, jtallen83 said:

Try a little chamber polishing. Flitz on a chamber mop and chucked in a drill. It will highlight any irregularities in the chamber as well.

I will try this before any further testing. Thanks.

[shooterrex]

You need the proper buffer, spring, and gas port for this to run. Springco orange w/the heavy buffer. Gas port probably needs to be opened up a bit.

[Bruce]

16 minutes ago, shooterrex said:

You need the proper buffer, spring, and gas port for this to run. Springco orange w/the heavy buffer. Gas port probably needs to be opened up a bit.

Thanks. The Sprinco orange spring arrived today and I have a #44 (.086) bit and #42 (.0935) bit on the way. Keeping my fingers crossed.

[98Z5V]

2 hours ago, Bruce said:

Thanks. The Sprinco orange spring arrived today and I have a #44 (.086) bit and #42 (.0935) bit on the way. Keeping my fingers crossed.

Don't drill the gas port up yet.  Hear me out.  You only change ONE THING at a time, and you test the gun - otherwise, you make a bunch of changes at once, and it's running WAY WORSE than it did before, and you have no idea what affected what.

Polish your chamber, shoot the gun.  Might sound crazy, but open that adjustable gas block WIDE OPEN, all the way up.  If you don't, you'll never know if that gas port diameter even meets the minimum, if that block isn't wide open.  Wide open, doesn't lock back?  You'll find that out with a few rounds (to verify), and you know that port diameter isn't enough.  Any other way other than that - and you're just guessing.

On 6/7/2021 at 5:40 PM, Bruce said:

Is the gas port large enough at .080”?

Is the spring length and coil count proper for a 7” extension?

Should I change to a 7-3/4” 308 carbine extension?

Should I try a Sprinco Orange buffer spring? (supposed to be made for 308 w/7” extension)

I'll answer these in order...

Is the gas port large enough at .080”?

I think the gas port diameter, at 0.080" is too big.  That's what I'm running on my .260 Rem AR, and it's perfect, and that thing has a 0.750" gas block journal size. Same barrel config for a .308 Win round, and I'm running a port diameter between 0.093"~0.096"...    Usually, down in caliber, down about 0.005" on the port diameter (per caliber).  UP in gas block journal diameter, down in port diameter, about 0.005".  And you're running a 0.936" gas block journal on that barrel.  That's down twice, from a 0.750" journal.  I think you need a port diameter around 0.065"~0.070".   Too late for that now, with a port diameter of 0.080"   Good thing you have an adjustable gas block.

Seems counter to what I stated before, doesn't it...   Just change one thing at a time...   Polish that chamber, shoot the gun.  THEN open that gas block up wide=open, shoot the gun.  2~3 shots is all you're gonna need to see, and record the gun's reaction...

Is the spring length and coil count proper for a 7” extension?

Nope.  Not for what you have in there right now.

Should I change to a 7-3/4” 308 carbine extension?

I will ALWAYS advocate the use of the Real Armalite AR-10 Carbine Recoil System parts, over anything else out there, every time.  There's a reason, too - it's not made by a million aftermarket companies, that can label any generic part with a name, and say "it's THIS."  It's made by one company, everytime, the same way, and it's THE proven recoil system for these guns.  You can shift a little one way or the other with some of the parts, but that's for another discussion.  You get a known, verified receiver extension that is truly 7 5/8" internal depth, and you use the Armalite EA1095 recoil spring in it...   Buffer needs to be 3.250" long, and H3-weighted (5.4oz)...

Should I try a Sprinco Orange buffer spring?

You have it  - it arrived today.  It IS the only spring made for the .308AR platform, running AR15 carbine receiver extensions, that are 7.000" internal depth, and you are stuck with running 2.500" long shorty buffers...  The ONLY spring, from it's ground-up design...

Might as well throw it in there with that 2.500" long 5.6oz buffer, and see what happens.  Gas block is wide open, new recoil system, shoot a few rounds and record what the gun does. 

After those results, we figure out what to do with your gas system, on this .243 Win AR... 

Edited June 9, 2021 by 98Z5V

[Bruce]

@98Z5V  I came here for sound advice and knowledge and everything said so far makes perfect sense. I'll polish the chamber, add the Sprinco orange spring along with the 5.6 oz buffer and fully open up the gas block. Next testing session is about 2-weeks away. Thanks again everyone.  

Edited June 9, 2021 by Bruce

[Bruce]

Brief update - Testing on this build has been delayed by an injury to my right hand. I expect to resume by the week of July 26th.

[98Z5V]

11 hours ago, Bruce said:

Brief update - Testing on this build has been delayed by an injury to my right hand. I expect to resume by the week of July 26th.

No worries, man - I ended up with a good pinch on my right hand, too.  Made lots of blood, and glad it was just skin, not muscle, tendon or bone.  It's doing good this week.

[Bruce]

Final update - I polished the barrel chamber with Flitz, not to a mirror finish but enough to make it noticeably shiny. Added the Sprinco orange buffer spring along with the  HeavyBuffers CAR-10 2.5”, 5.6oz buffer. Opened up the Superlative Arms adjustable gas block, ran 20 rounds of Sig 55 grain Varmint & Predator through it and......................it WORKS!! I ended up closing the gas block 1/2 turn to get a very nice 3 o-clock ejection pattern and my brass doesn't even touch the brass deflector. No shoulder dents, no bent rims and no sooty brass. Last round BHO every time with MAGPUL mags.

Thank you so much to everyone that commented on this project especially @98Z5V !! I can honestly say that I would NOT have ever figured this out without your help.

[Armed Eye Doc]

That is great to hear!  I'm glad it is working well.  Thanks for coming back to update.

Now it's time to start on the next build.

[shooterrex]

Glad it's finally running.

[98Z5V]

@Bruce - awesome news, man!!!     I'm glad this beast is running.  More than you know.  Keep the updates running, on how it's doing.  This is fantastic news. 

[sagebrush]

another satisfied customer