Ammo issues with my 308 AR

[b rad]

So I took my 308 AR out Sunday and fired 9 rounds. On the 10th, the round chambered but the bolt wouldnt go forward all the way. I looked into the chamber and found several small metal shards just inside the lugs, so I dug them out and realized it was a primer that had exploded. After looking through the 9 pieces of fired brass it turns out that 3 of them had no primers left and the other 6 had slight signs of overpressure. The ammo was new Southwest 175gr MatchKing and the gun was an Iron Ridge billet upper and lower with a 16" mid length Rainier Match barrel that I had assembled by a local gunsmith. I guess my question is whether it was caused by the ammo or the gun, and what might the reason be. Improper head spacing would be my first guess but I wanted to run by you guys to get some other thoughts. Thanks

 

[Shoottothrill]

Sounds like the ammo is too hot. I would run some FGMM 175's and see how it does. Gas guns can be finicky and need just the right amount of gas to function smoothly. That ammo might be fine in a bolt gun but in an AR 10 could be way over-pressure.

 

[leadbullet]

I have had to down load my 308 ammo from bolt to gas gun, blown primers and all kinds of issues, once I got it sorted the gas gun ran perfectly and I just needed to keep the 308 loads separated
Cheers.

 

[BCP]

So I took my 308 AR out Sunday and fired 9 rounds. On the 10th, the round chambered but the bolt wouldnt go forward all the way. I looked into the chamber and found several small metal shards just inside the lugs, so I dug them out and realized it was a primer that had exploded. After looking through the 9 pieces of fired brass it turns out that 3 of them had no primers left and the other 6 had slight signs of overpressure. The ammo was new Southwest 175gr MatchKing and the gun was an Iron Ridge billet upper and lower with a 16" mid length Rainier Match barrel that I had assembled by a local gunsmith. I guess my question is whether it was caused by the ammo or the gun, and what might the reason be. Improper head spacing would be my first guess but I wanted to run by you guys to get some other thoughts. Thanks

That sounds like an ammo problem to me. I would call Southwest and tell them.

 

[hotbrass]

Check the length of the firing pin protrusion and see if it is in spec. And check your headspace.

 

[Shoottothrill]

I don't think Southwest has really developed a "Gas Gun Load" I think they load most of their stuff on the warm side and some AR10's just don't like it so much.

Southwest if I'm wrong apologies. Great guys over there, they even gave me some samples to try in my Gap 10 and I have yet to have time to get out and shoot them.

 

[b rad]

I put a box through my M1A and didnt notice any problems, but these were the first ones through this particular gun. Even if it was loaded hot, how could it blow the primer completely out of the case if the bolt was seated all the way? Thats what makes me think its a headspace issue. The primers on the other rounds were a little flat but the firing pin didnt pierce any of them. Ill take some pictures this afternoon after work and put them up. Thanks for the help

 

[b rad]

ammo pics

 

[Shoottothrill]

Hmmm. Never seen the primer completely gone.

 

[alsation]

Friend, you have pressure issues, blown primers are a classic example of too high pressure. Plus looking at your photo the other fired primers are cratered around the firing pin indentation, not good. You're not doing your rifle nor your face any favors using this ammo in your rig.


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[BCP]

ammo pics

Those are classic pressure signs. See the shiny scratch mark on the case head going from the primer to the rim? That is where it leaked and a bunch of hot gas came out @ 60k PSI. Also it doesn't blow the primer out with the bolt seated all the way what happens is the primer pocket gets loose and then when it cycles it tosses the primer out too. It can land in the extension or in your magazine or go out the ejection port.

What is probably happening is that those rounds are loaded hot and you have a very tight chamber on your 308 AR so you're hitting pressure sooner.

 

[Rust]

First impression is the loads are just plain too hot. Another contributing factor could be the short barrel. With the 175s and a powder like RL-15 or slower, gas pressure is likely pretty high at the gas port causing the bolt to open while chamber pressure is still high. That can cause all sorts of weird stuff. You might want to check hardspacing too, it could be too much to begin with and really hot loads could stretch things too.

Inspect your bolt and carrier very carefully for flame cutting of the bolt face or any deformation of any component. Dump the match loads. Lot of things that work in a bolt gun won't work in a semi, and somee things that work in a long barrel semi won't work in a short barrel semi.

 

[KYbulldog]

Sounds like the ammo is too hot. I would run some FGMM 175's and see how it does. Gas guns can be finicky and need just the right amount of gas to function smoothly. That ammo might be fine in a bolt gun but in an AR 10 could be way over-pressure.

Pull the bullet, weigh the charge and ask SWA what they used to load them with.(fwiw, Armalite rifles warranty is voided with use of reloads or hand loaded ammo- suggesting that they are completely fine with commercial ammunition pressures.) I have shot all kinds of commercial ammo and have never noticed the problems you have experienced. There is one brand I'll never buy again, and that is MFS/Ruag…it was cheap and I got what I paid for.
Now, SWA says their ammo is "ideal for both bolt and semi-auto" see below.

308 175 Sierra Match King Premium, New Lapua Brass

Ideal for both bolt and semi-automatic rifles.

-308 Winchester
-175 grain Sierra MatchKing
-2650 fps from a 24" barrel
-G1 BC: 0.496
-20 Round Box

That looks like a pretty standard fgmm clone load.

As far as the Ejector swipes go, my impression is that those are kind of par for the course in 308(semi's)ARs. Sometimes I get them and sometimes I don't. I have loaded my ammo to the point of very slight cratering, and minor flattening of the primer, the ejector swipe is no more prevalent in those hotter rounds than they are with the lighter loads.

If your chamber checks out, I'd get some Lake City 7.62, SSA 7.62, FGMM 308, and some other decent brand Commercial 308, and see if you have the same issues.

Also consider getting an SLR Adjustable Gas Block for your rig.

 

[b rad]

I have some factory FGMM and AMAX that Im going to try in it, I just didnt have it out there with me when this happened. I almost went with an adjustable gas block when I was getting it together but I talked myself out of it. Kicking myself for it now. Ill try to come up with a scale and weigh the charge from a few rounds to see if they might have been a little too hot. Thanks again for the help

 

[mtrmn]

The ammo is too hot, headspace wrong OR the gun is overgassed and unlocks the bolt too soon while the pressure still has the case welded to the chamber. Are any of the case rims bent due to the extractor? I'm leaning toward overgassed since it has a 16" bbl. The adjustable gas block sounds like the cure to me but by all means get the headspace verified. Try that ammo in another gun or try different ammo in yours.

 

[Speeddog]

I had the same problems with SWA 175s in a Les Baer Monolith; also with Federal Match. Not surprising since they are similar loads. This is NOT to disparage SWA: I've purchased thousands of rounds of ammo from them in a bunch of calibers. Their .308s shoot well in my bolt guns so I've relegated them to that role. Les uses very tight match chambering and I suspect that's the culprit in my case.

George Gardner has talked about gas guns and their ammo needs in a number of threads and often mentions Copper Creek gas gun loads as a solution. Another is custom ammo from Adam McCourt: he builds me 175SMKs that work and shoot beautifully in my AR. I'd strongly recommend his work to anyone looking for an ammo solution.

Hope this helps

 

[smschulz]

I had the exact same problem on my MA TEN's (all with RA UM barrels 20,18,16) .....using Black Hills Match 168 and Nosler CC 175's.
No problem in the bolt gun (5R).
It is just too hot.
That is why I load all mine now.
43.0~43.5gn Varget & 168gn SMK's makes a sweet load for my gas guns now.
More accurate than FGMM.