DPMS AR-10 problem

D V US · Jan 4, 2016

I have a gen 1 DPMS .308 SASS that has developed an issue. I was running through a mag of 168gr. BTHPs and the first 10 rounds cycled normally, but then it started to fail to feed. I assumed it was a fluke but the next three shots got worse before checked my brass. Wow was I lucky. The first seven cases were fine, 8 & 9 started showing head swiping from the ejector, 10 showed swiping and a slight burn on the case head and 11 had a larger burn on the case head. 12 - 14 had the primers completely ejected and heavy scarring on the case head. No primers in the other cases showed any overpressure signs.

The specifics:
DPMS SASS .308 18" barrel Gen 1
Approx total round count ~250
Load data
168gr. Hornady BTHP
42.2 gr. H4895 (midrange load)
Winchester primer
various cases
All charges were individually weighed and loaded with an o.a.l. length of 2.800, full length resized and loaded with Hornady dies.
Rounds were fired slow fire, about 1 minute between shots and five minutes between 5 shot groups. Temperatures around 45 degrees.

Any help in identifying this problem would be appreciated.

aviator41 · Jan 4, 2016

you might try retaking your photos so that the primer end of the cases are in focus, not the background. It's hard to see what's going on with what you've got posted.

However my first guess is Firing Pin protusion is WAY out of spec. that will cause the drag marks your seeing as well as the primers unseating. if you don't have a FP protusion guage, you might consider getting one, or at least a good set of calipers. either can be used. Between excessive FP protrusion and a bolt at the limits of headspace, then wearing in as you shot it, you get the results you've listed.

This is not all that uncommon with DPMS bolts. Their tolerances are a bit "loose" by most standards.

Make some reduce rounds and see if the problem diminishes or goes away. if not, you may have to get the bolt bushed. or replaced. Since you're shooting handloads through it, your warranty is void so be prepared to pony up the dough.

Jwryan84 · Jan 4, 2016

Look a lil hot to me

Sanford · Jan 4, 2016

If I'm understanding correctly these were all fired in progression out of the same loaded mag so I'll hazard a guess that a loose neck or inadequate crimp allowed bullet setback to create increased case pressure; the deeper in the magazine the more shock from firing, more setback, and higher pressure.

technetium-99m · Jan 4, 2016

Your popped primers are out of military brass from what I can tell. Loads that are okay in commercial cases are hot in military brass. I'm betting your load is too hot for those cases. 42.5 grains of anything behind a 168 in military brass is getting spicy.

technetium-99m · Jan 4, 2016

Also every 308 I've had was way overgassed. Adjustable gas blocks help a lot when it comes to getting LR308s to run 100%.

D V US · Jan 4, 2016

Bullet setback due to a light crimp/recoil is a no go, I suspected that myself at first. All remaining cartridges in the mag still measure out to 2.800 the same as when loaded. The last cartridge chambered as well had no setback.
I have been reading on other forums about the over gassing issue and how it is a common problem in DPMS .308s. They are showing pics that look about like mine as far as case head damage. Gonna have to look into an adjustable gas block.
Yes the blown primers are in once fired military brass but i also have some of the same case lot fired with the same load that showed no damage whatsoever. I would have doubts with my powder measure if I hadn't weighed each charge.
I'm suspecting a mechanical issue, not a load issue, but I will try it with some known factory ammo that I've used in it before and see what happens.

D V US · Jan 5, 2016

Ok, I went to the range and ran a mag of factory 150 gr rounds through the rifle. At first everything was fine and then after ten rounds it started to do a little ejector swiping again but nothing like before and didn't damage the brass much at all. Finished the mag with no problems. Loaded a mag of the ammo that started the problem (ALL in military brass). First shot, perfect. Second shot same, third shot showed significant swipe and fourth ejected the primer. Talked with my reloading guru and showed him the cases. He pretty much came up with the same thing I did, that the load itself is good with no pressure signs on the primers, but the heavier bullet is just enough to overgas the action and open it too early.

D V US · Jan 14, 2016

technetium-99m said:
Also every 308 I've had was way overgassed. Adjustable gas blocks help a lot when it comes to getting LR308s to run 100%.

How right you are! Drilled and tapped my gas block for an adjustment screw and cured all the problems once it was dialed in. I was even able to run a couple heavier loads than what had given me problems initially. Thank you all for your help and input. :thumbup3:

konan · Jan 19, 2016

Is your brass trimmed before reloading? 2.015 is listed as max length but causes pressure problems in three of my blackhole weaponry barrels that have tighter match chambers. I trim everything to 2.005 ...no problems. Brass that is too long will create pressure issues like blown primers and ejection issues.

DPMS AR-10 problem

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