I recently built a 300 Blk Out upper. I used a CMMG barrel and bolt carrier group in the build. I also stripped the bolt and did a headspace check. GO-GAGE closed easily and NOGO-GAUGE refused to close even a little. I took the build out for test firing. First I used some PNW 147 gr fmj and the bolt only partly unlocked and jammed. After extracting the empty from the chamber. I noted a really flat primer and the case wall showed a large amount of scratching. I tried one more round and this time the case was extracted, but the bolt short stroked. Again the primer was flattened and case wall extremely scratched. Two additional rounds did the same. I then switched ammo to Gorilla 200 gr subsonic. Ten rounds ran thru the build with no problem. I then tried 20 rounds of Armageddon 110 gr. All 20 rounds went thru the build with no problem. I have since the range time stripped the bolt again and checked headspace ( no problem), checked the ammo with the stripped bolt (none of the rounds stuck in the chamber including the PNW 147 and none of the rounds showed the bullet marked by the rifling ( no indication of short leadin). I shined a bore light down the bore, and the chamber and rifling show no problem. At this point I can only assume that the PNW is overloaded and this is the problem.

My question is. Am I missing anything to check?

Thanks,
Mike

cant read one gigantor sentance

I hope you didn't buy the range box of ammo -- I would stay away from that PNW ammo for now.  I'd try contacting PNW and explain the problem and that you can run other ammo just fine. See if you can find a Lot number on the box so they can check to see if there were any other complaints. Just my 2 cents.

Did you get the ammo from Dollar General, I've never heard of any of those.


Concerning the scratched cases, is it from the magazine lips or is there a burr somewhere?

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Sorry to waste your time.

Bought all the ammo from CTD and Midway. I'll try to contact the manufacturer and see what's up.

Thanks,
Mike

AS I noted I bought all the ammo from CTD and Midway' The scratches are over the entire surface of the case, like the pressure is so great the case is imprinting the tool marks in the chamber and extraction causes the scratch marks.

Thanks for the reply;
Mike

What length gas system do you have on that rifle?  What length barrel?  (16" barrel with a pistol gas system?)

With the scratches and the stretching of the cases, it sounds like the rifle is cycling a bit too quickly with the hotter round.   ie. trying to extract the case before the chamber pressure has dropped sufficiently.   Are you also seeing bending of the case rim where the extractor is pulling on it?    I believe this would also correlate to the bolt jam and the short stroke issue.   Too much energy is being spent trying to pull a case that this still expanded against the chamber walls.

If you want to shoot the hotter loads, I would look into slowing down the cycle timing with a heavier buffer, longer gas tube, (one of the ones that wraps around the barrel) or possibly an adjustable gas block.

The flattened primer, while it indicates the pressures are up there, is not necessarily an indicator or over pressure or an ammo problem, especially if the primer pocket is on the high side of its size tolerance.   When the primers start to extrude back into the firing pin hole is a good indicator things are getting a little too hot.

Some of the others with more experience may have other ideas or thoughts.     Just the first things I'd look at.

I have a 16" bbl. with a carbine length gas tube about equal to 9"  length.
I checked back on Midway about this ammo and there have been 4 customer complaints  on high pressure. I guess I'll just stay away from this ammo brand. Everything else functions fine in the build. I was afraid I had done something wrong on the build. Thanks for the info.
Mike

Welcome to ARFCOM.  (I've gotta agree that your post is hard to read - the old "wall of text" thing gets in the way of getting your message across.)

A new gun, in any caliber, needs quality, full-power ammunition to break in all the moving parts.  It's not a good idea to go cheap with ammo at this point in the gun's life.  Not that PNW seems to be "cheap," but this was the first I've ever heard of them, and apparently that's true of a bunch of folks here.  Sticking with a reputable brand, one that has a track record of getting it right with your particular caliber, is a very good idea for your first time out - and for a while once you get it broken in.

A friend of a friend [sic] picked up some off brand BLk ammo not to long ago for a "Good" deal (not really as far as BLK ammo goes) & had all kinds of issues.

Right off it was blowing primers. Some were showing gas around the primer, others were totally gone. After about 50% failure of only 50 or so of the 500 he bought he took them to my friend that is a ammo manufacturer. He tried a few & found about the same failure rate in another barrel. After that the ammo was thoroughly tested & taken apart.

They were using a mix of Lil Gun under 150FMJ's. The case fills were eradic varying by as much as a grain & a half. Not a big deal it might sound but with the loads in this cartridge it was a big deal. Also some of the cases were not fitting the case guage perfectly around the shoulder & neck.

The cheap ammo was truly "cheap" on all counts. Buyer beware!