tuning large frame AR to prevent case head swipe

[jnr2233]

I found a load that is very accurate in a particular .308 AR, however, the fired cases all show signs of case head swipe. I’m not seeing any other pressure signs. The load is 44.6 grains of 8208xbr with 155 smk in lapua cases with wlr primers. I would like to sort this out because the lapua brass is expensive.

Barrel - jp lightweight 18” rlg
Lmos carrier with jp high pressure bolt
Sa adjustable gas block
Jp scs

I’m using 3 tungsten weights on the scs along with the heavy recoil spring and I’ve adjusted the gas block. I think my next step is to switch to a full mass bolt carrier but I wanted to see if any other options are available before spending more money. Any ideas?

 

[LawnMM]

Get rid of the lightweight carrier

 

[AMGtuned]

Too stiff of ejector spring? I know I've heard of people shortening them to relieve tension.

 

[bfoosh006]

Get rid of the lightweight carrier

This... slowing the bolt unlocking should help.

 

[MSTN]

Ditch the low mass carrier for the full mass version.

Find the lowest gas block setting that will lock the bolt to the rear on the last round fired from the magazine. Increase the gas just one click over that.

This gas setting typically gives optimum accuracy with decent reliability. It also yields the minimum usable bolt carrier velocity, which also means slower unlocking of the bolt, giving the case time to "relax". This should reduce your swiping problem.

 

[KTAK]

Drop the powder charge a whole grain and see what happens.... no money spent if that's the problem. I found that I had to drop down all my bolt gun loads when using the same rounds in my LR308

 

[newguy2k3v2]

If LMT bolts are compatible with JP I noticed a night and day difference in ejector swipe switching from the gen 1 single ejector to the dual ejector bolt.

I can only assume that since the dual ejectors more or less just contact around the rim the lower or lack of pressure there means less swipe.

 

[NukeMMC]

My 308 Mega upper has a 20" Proof steel barrel, Aero carrier and JP bolt with JP SCS. No swipe issues even running suppressed (TBAC Ultra 7). Adjusted the SLR gas block with the can, since that's how I will normally shoot it. I figure I will open it up 2 clicks with the can off and brake on (419 Hellfire system). I am running 175SMKs with 42.7/RE15 or 42.2/4064 in LC brass to give me the same performance as FGMM175.

Try a heavier carrier. My 6.5CM upper for the same rifle uses a JP VMOS with 2 tungsten weights and the same SCS (same lower). No swipes there even with the can.

 

[MSTN]

I can't figure out WHY you're having this problem with the components you're using. The LMOS has always worked perfectly for me with the .308. An 18" with rifle length gas should be smooth as can be with this combo.

Could it possibly be the "heavy" spring is marked wrong in your SCS kit? Get an H2 buffer and a .308 carbine recoil spring for an inexpensive way to check this out.

How far have you dialed down the gas?

And what type ammo are you using? If the answer is FGGM, their brass ALWAYS does that, and is way too soft for gas gun use.

 

[jnr2233]

I can't figure out WHY you're having this problem with the components you're using. The LMOS has always worked perfectly for me with the .308. An 18" with rifle length gas should be smooth as can be with this combo.

Could it possibly be the "heavy" spring is marked wrong in your SCS kit? Get an H2 buffer and a .308 carbine recoil spring for an inexpensive way to check this out.

How far have you dialed down the gas?

And what type ammo are you using? If the answer is FGGM, their brass ALWAYS does that, and is way too soft for gas gun use.

I can't figure out WHY you're having this problem with the components you're using. The LMOS has always worked perfectly for me with the .308. An 18" with rifle length gas should be smooth as can be with this combo.

Could it possibly be the "heavy" spring is marked wrong in your SCS kit? Get an H2 buffer and a .308 carbine recoil spring for an inexpensive way to check this out.

How far have you dialed down the gas?

And what type ammo are you using? If the answer is FGGM, their brass ALWAYS does that, and is way too soft for gas gun use.

I get the feeling that this should work too.

Ammo: The load I’m shooting is a 155 smk over 44.6 grains of 8208xbr (average muzzle velocity is 2775 FPS), however, I’ve shot lots of fgmm 168s and they get case head swipe also.

Gas block adjustment: I used the recommended procedure, I.e., open it until the bolt locks back on a single round and then one or 2more clicks



Unfortunately, It looks like I’m going to have to buy a full mass carrier unless I want to back off on the charge (which I don’t).

 

[MSTN]

That's a pretty hot load. What kind of brass are you using?

You've got a FMOS in your future, I'm guessing.

 

[jnr2233]

That's a pretty hot load. What kind of brass are you using?

You've got a FMOS in your future, I'm guessing.

I’m using lapua brass. The load is getting hot but it’s under the published max and I’m not seeing any other pressure signs.

 

[Constructor]

I found a load that is very accurate in a particular .308 AR, however, the fired cases all show signs of case head swipe. I’m not seeing any other pressure signs. The load is 44.6 grains of 8208xbr with 155 smk in lapua cases with wlr primers. I would like to sort this out because the lapua brass is expensive.

Barrel - jp lightweight 18” rlg
Lmos carrier with jp high pressure bolt
Sa adjustable gas block
Jp scs

I’m using 3 tungsten weights on the scs along with the heavy recoil spring and I’ve adjusted the gas block. I think my next step is to switch to a full mass bolt carrier but I wanted to see if any other options are available before spending more money. Any ideas?

In order to get swipes the case must be stuck to the chamber walls while the bolt turns.
Either you are getting too much gas making the bolt unlock before the case has a chance to shrink or the case is stuck to the chamber walls because too much pressure or a sticky or rough chamber.

Try adjusting the gas down to where it just locks back for the best accuracy.
Hone or polish the chamber, chamber flex hone or Iosso bore paste it's a little more abrasive than flitz or JB.

 

[bfoosh006]

Make sure the ejector is "burr" free on the tip as well. Just a thought.