Gunsmithing DPMS LR .308 & Excessive Pressure

[Ishallbie O'Cullkillin]

Friend has a LR.308, 24" barrel that displays excessive pressure signs with anything over 42gr. Varget, 168 Bergers.Very accurate with this load.Have not chrono'd them yet. He has them seated out to mag length. Seems like he should be able to load a bit more into the Fed brass, My Armalite handles 44gr easy. I told him that he could have a rough bore, or that his carrier, buffer or spring might be too light, or that the gun is overgassed. Any thing else it could be?

 

[918v]

Re: DPMS LR .308 & Excessive Pressure

FC brass + tight chamber = high pressure. Tell him to try Winchester brass.

 

[Falar]

Re: DPMS LR .308 & Excessive Pressure

By pressure signs are you referring to ejector swipe?

 

[cav has been]

Re: DPMS LR .308 & Excessive Pressure

A friend of mine also had similar problems with his DPMS. It was diagnosed by others as high pressure, my take was rough chamber. Upper was sent back to DPMS, they evidently had a laying on of hands, an exorcism, or did some of that gunsmith mumbojumbo, and a week later he had his upper back. It now operates as it should with out signs of pressure or rough chamber. Someone at DPMS did fess up that they had polished the chamber a bit. On a good note the repairs were completed in a timely manner and there was no charge.

 

[lvcatfish]

Re: DPMS LR .308 & Excessive Pressure

My LR308 had big issues with eating 1X FC brass. Brass switch gained me more powder/velocity and less brass damage.

 

[Pat M]

Re: DPMS LR .308 & Excessive Pressure

My 260 upper was punching primers like mad, sent it in , quick turn around and is fine now. Still want an adjustable gas block.

 

[gobshite]

Re: DPMS LR .308 & Excessive Pressure

Please describe ejector swipe- mine seems to blow a plug of brass into the ejector pin bore in the bolt face, which then rotates, shearing it off because the brass doesn't rotate with the bolt. Is this what a rough chamber does?

 

[gobshite]

Re: DPMS LR .308 & Excessive Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gobshite</div><div class="ubbcode-body"> Please describe ejector swipe- mine seems to blow a plug of brass into the ejector pin bore in the bolt face, which then rotates, shearing it off because the brass doesn't rotate with the bolt. Is this what a rough chamber does?
</div></div>Double the powder and shorten the fuse!- Captain Hook

 

[Falar]

Re: DPMS LR .308 & Excessive Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gobshite</div><div class="ubbcode-body"> Please describe ejector swipe- mine seems to blow a plug of brass into the ejector pin bore in the bolt face, which then rotates, shearing it off because the brass doesn't rotate with the bolt. Is this what a rough chamber does?
</div></div>

Here are some cases with swipe:

As far as I know, it has nothing to do with a rough chamber. In another thread about reloading for ARs I posted the Armalite tech note explaining what causes this and why it is not USUALLY a cause for concern (unlike in bolt rifles) and is common in ARs.

 

[Pat M]

Re: DPMS LR .308 & Excessive Pressure

Both my 308 and 260 upper brass look like that.

 

[gobshite]

Re: DPMS LR .308 & Excessive Pressure

I had a few loose primer pockets, also, with both Winchester and Federal brass, upon the first firing. Wish I had photographed the damage so everyone could read my brass.

 

[Goin'Hot]

Re: DPMS LR .308 & Excessive Pressure

My brass looked worse than Falars before I put an adjustable gas block on it. I recommend a block on the DPMS LR308's.

 

[Falar]

Re: DPMS LR .308 & Excessive Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Goin'Hot</div><div class="ubbcode-body">My brass looked worse than Falars before I put an adjustable gas block on it. I recommend a block on the DPMS LR308's. </div></div>

I like the one JP makes, but unfortunately I'm the odd man out with my barrel OD of .875". They make a .750" and a .936" model. I tamed it a bit though by adding a heavy buffer and the Tubb CWS. I have an AR10 but the principle should be the same across the various .308 ARs.

 

[Unknown]

Re: DPMS LR .308 & Excessive Pressure

I have a DPMS 24 inch 308 and found that the throat on my rifle is pretty short. If I have the bullets seated to 2.815 (a bit long), I can see where the lands begin to engrave on the ogive of the bullet with the naked eye. I'm just guessing that DPMS did this so that as the throat erodes, I will be able to keep chasing the lands by seating the bullet out a bit farther forward.

So, for now (at least with the bullet I'm currently using) I seated the bullets to 2.800 and they are off the lands.

I'll have to recheck it when I switch from the Sierra 155 (old palma bullet) back to the Laupa Scenar, or the Lapua lock base.

 

[skurtz]

Re: DPMS LR .308 & Excessive Pressure

I have 24" LR308 cut down to 21.5" and the barrel diameter reduced (.750)for shooting Heavy Metal. It used to really have ejector swipe until I installed a JP gas block. It reduces it with the 168 grain loads but not much with the 125 grain loads. The primers do not show a lot of pressure with either load.

 

[Ishallbie O'Cullkillin]

Re: DPMS LR .308 & Excessive Pressure

Hey Gobshite, maybe your chamber is shorter than what your magazine length allows. Are you able to extract an unfired handload without yanking the bullet out of the casing?

 

[Falar]

Re: DPMS LR .308 & Excessive Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Unknown</div><div class="ubbcode-body"> I'm just guessing that DPMS did this so that as the throat erodes, I will be able to keep chasing the lands by seating the bullet out a bit farther forward.

</div></div>

I don't think many manufacturers think about handloaders at all. More than likely they have short throats so that factory ammo (which will be 2.8" for the most part) will not be jumping as much in an attempt to increase accuracy.

 

[CS95]

Re: DPMS LR .308 & Excessive Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Falar</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Unknown</div><div class="ubbcode-body"> I'm just guessing that DPMS did this so that as the throat erodes, I will be able to keep chasing the lands by seating the bullet out a bit farther forward.

</div></div>

I don't think many manufacturers think about handloaders at all. More than likely they have short throats so that factory ammo (which will be 2.8" for the most part) will not be jumping as much in an attempt to increase accuracy. </div></div>

This

 

[RampageOTB]

Re: DPMS LR .308 & Excessive Pressure

Big Bore - glad I found, I'm thinking about turning down my LR308 too, maybe to .875, or leaving it .936 under the block and down to .82 or so in front. Did you research the process before doing it, or notice any effect on accuracy after doing it? I'm worried that if the barrel wasn't stress relieved before rifling, that turning it down will alter the bore dimension slightly (no personal experience, just some reading I've done on that effect).
Thanks!

 

[toygunner]

Re: DPMS LR .308 & Excessive Pressure

I turned down my 24" bull to 20" plus the comp. It was .880 behind and in front of the gas block. There was no loss of accuracy. I just made sure the barrel did not get hot during the turning process. I took small cuts.

 

[RampageOTB]

Re: DPMS LR .308 & Excessive Pressure

Thanks, that sounds like a very reasonable build. About how much did it lighten it?

 

[Goin'Hot]

Re: DPMS LR .308 & Excessive Pressure

Why don't you drill and tap the block that's on the rifle now? That's what I did. My brass looks great with the gas turned down.

 

[skurtz]

Re: DPMS LR .308 & Excessive Pressure

Check my PM.
Mine is turned just over .750 under the handguard, just enough of a step for the gas block to set against. I would measure it but its hell to get to now. It also has a carbon fiber handguard which really drops the weight. Weight wise it is just a little heavier than a heavy barreled AR15 in .223.

 

[1911fan]

Re: DPMS LR .308 & Excessive Pressure

Pic?


1911fan

 

[fastford]

Re: DPMS LR .308 & Excessive Pressure

tag

 

[TRAAV]

Re: DPMS LR .308 & Excessive Pressure

I am having the same problem with my new 6.5 creedmoor. It is blowing out the primmer and leaving a big gash in the case. I'm going to see my gun guy tues. I will let ya know what he says :-/

 

[TRAAV]

Re: DPMS LR .308 & Excessive Pressure

He said I'm just over pressure. My gun checked out fine as far as head space and coal. He said i could have a tight bore and need to check my fps so i can get my pressure. I guess i will go from there.