Primers popping out, could it be the gun?


Tim James
January 16, 2009, 08:14 PM
Just tried out my CMMG mid-length gas 16" AR-15. Put the lower together myself with a JP trigger. Plopped the upper on there after a little bit of lubrication (but I didn't do a whole lot since I'm new to these rifles). I was shooting some American Eagle Tactical XM193, all I could find locally. Everything was fine for 20-30 rounds until I got a failure to fire. Extracted it and didn't notice a primer strike, then noticed that a little piece of primer flew out with it. Tried a some more rounds and noticed a few primer shards just behind the chamber there between the lugs.

Dangerous? Something wrong with the rifle? Hoping to get it out to the 3-gun match in the morning but it's not looking good at this point.

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January 16, 2009, 08:20 PM
is the bolt unlocking before the pressure is down, is barrel headspaced correctly?

January 16, 2009, 08:23 PM
I use the same ammo and have for a year or so.They did have a bad lot of it causing the same problem and some serious jams. I'd call Federal on Monday and have them check the lot ## so they can replace it if it is suspect.

January 16, 2009, 08:31 PM
It could be the American Eagle. Their brass has excessively large primer pockets in .45 caliber, and I have personal experience with primers falling right out of once-fired American Eagle brass. Same problem could carry over to .223?

Tim James
January 16, 2009, 08:33 PM
is the bolt unlocking before the pressure is downHmm, something to do with the mid-length gas system, perhaps? I'm not sure how to check this.

It's 5.56 by the way.

January 16, 2009, 08:38 PM
i'd try different ammo before i pointed a finger at the rifle.....

Tim James
January 17, 2009, 01:46 PM
Looks like it was the ammo. Happened again within 20 rounds at a 3-gun match. Jammed the gun up significantly, we had to tap the bolt forward a tad with a socket wrench extension. Used someone's reloads for the next stage, 40 rounds flawless.

Fred Fuller
January 17, 2009, 03:19 PM
Thus the crimped primers on military ammunition...


January 17, 2009, 05:19 PM
One other thing to check (sounds stupid to mention, but...)

Make sure your barrel is chambered for a 5.56 NATO and NOT a 223 Rem (confirm this the manufacturer if your not sure) normally it's stamped into the barrel somewhere, BUT I'd confirm anyway.

AE XM-193 are M193 Spec

Popping the primer can be a sign that the round is overpressure, since the 5.56 has a longer Leade, than the 223 REM SAAMI this can boost the cartridge pressure (which is higher for the NATO round anyway) since the round has less expansion space at the bottleneck and cause this primer pop and foul the action at best. Worst... well I'll leave that to the imagination.

If you're firing 223 Rem rounds consistently, but have issues with 5.56 consistently, you're likely chambered for a 223 Rem.

Tim James
January 17, 2009, 10:32 PM
Yeah CMMG clearly advertises as 5.56 but who knows if they're actually made right.

January 17, 2009, 10:40 PM
But, then how many people know the difference between 5.56 MIL-SPEC and 223 SAAMI...?


Give them a call/mail/, see what they say (if only for peace of mind). Giving CMMG benefit of the doubt it could be the ammo, XM-193 is normally very high quality, but you could have had a Friday Afternoon batch.

January 17, 2009, 11:26 PM
"...Thus the crimped primers on military ammunition..." Those are for using the ammo in MG's.
"...Used someone's reloads..." That's not a good idea either, but it does prove the primers blowing is caused by the ammo. Federal brass is known for being soft.

January 18, 2009, 12:00 AM
AE XM-193 are M193 Spec

if they were "mil spec" they would be M193 instead of XM193

they are factory seconds, and different lots are discarded for different reasons, which makes it fairly difficult to broad claims about all lots of XM193 with any degree of confidence.

I have read numerous reports of some lots being overpressured. And in most cases, it seems to be a combination of out-of-spec chamber and out-of-spec ammo causing it to poop primers. go figure.

my advice would be, assuming you want to keep shooting this ammo, is to do a search and a little reading on "ned's chamber reamer"

January 18, 2009, 01:29 PM
Check the primer pockets - are they out of round or obviously too big? Did the case head deform? That could indicate an oversized chamber at the back end.

Overpressure usually results in flattened deformed case heads. The primer flows to fill the pocket flush with the case head and there is no little beveled ring around the edge.

Another thing to check is for a sticky chamber. Primers normally back out a bit and then the case pushes back onto them. A sticky chamber could be the cause.

This could also be a classic .223 chamber / 5.56mm ammo combo problem where the neck sticks in the chamber and the case does not set back properly. I've experienced that but had FTE not popped primers.

Try cleaning the chamber thoroughly and carefully with a good solvent and a chamber brush. I've chucked a GI cleaning rod in a hand drill with a GI chamber brush on the end, sprayed WD-40 liberally, and spun it in the chamber to polish it a bit. Helped a bit.

Also try different brands of ammo.

Tim James
January 18, 2009, 09:37 PM
Primer pocket looks okay, just a little powder residue. The head does not look flattened but there is a slight unevenness as if the extractor ripped the rim out too hard. You can see a bit of warping on the head stamp at the same place. Maybe a hint of similar warping directly opposite on the other side of the case.

Just pulled the handguards off and it's marked 5.56.

Having trouble locating headspace go gauges for 5.56. Seems like all the readily available ones are 1.4636", and in this case I think I would want to try with a slightly longer one to see if there is enough clearance to close it with some room for error.

Coal Dragger
January 18, 2009, 09:40 PM
Hope it's only an ammo issue, but given my experience with crap-tastic CMMG "quality" it very well could be the rifle.

AR-15 Rep
January 18, 2009, 09:46 PM
I have had the same problem with surplus ammo. There may be a problem with them being incorrectly loaded, so check the lot number with Federal. I never had a problem with the Federal American Eagle commercial brand, but with the mil surp quite a few with primers popping out.

January 18, 2009, 10:03 PM
Tim James,
"Having trouble locating headspace go gauges for 5.56. Seems like all the readily available ones are 1.4636", and in this case I think I would want to try with a slightly longer one to see if there is enough clearance to close it with some room for error."
Add a piece of tape to the back of guage & measure.
This guy does it at about 2 minutes into this video

Tim James
January 18, 2009, 10:20 PM
Two thousands might be just a bit too much. I think the military go gauge is 1.4646 or one thousandth longer. I actually want it to close on the 1.4646 I believe.

January 18, 2009, 10:43 PM
pretty sure you can get 5.56 (as well as 223rem) ones from brownells, but headspace is probably not the issue

January 18, 2009, 11:10 PM
.. it is a low pressure problem. To Gungnir. Think about it for a minute... how much pressure is generated in your chamber. 10,000 - 15,000 - 20,000 or higher C.U.P.'s or PSI (use either). If you re-load you know that the primer pops out with very little pressure.

Every time you fire a gun, the primer is popped out of it's pocket and forced against the bolt face. When the chamber pressure begins to drop the cartridge is then also pushed back into the bolt face, at that point it picks the primer back up. If the chamber pressure is too low, this will not happen.

I do not ghink this is what's happening to the OP, but I was responding to the comment or common belief that a popped primer is a "high" pressure problem.

Tim James
January 19, 2009, 11:27 AM
Do you not think it's the issue just because you think it's something else?

How would you check typical causes of a low pressure problem in a gun (assuming for a moment that it's not the ammo)?

Left a message with Federal tech support about this lot number. Also emailed CMMG for their opinion.

There is a PTG 5.56x45 headspace gauge but they don't indicate how long it actually is. Sent them an email. Even if I don't buy it or rule it out as something else, I'm just curious now.

Ben Shepherd
January 19, 2009, 01:33 PM
Take one of the rounds with a popped primer and see how easy it is to insert a new primer in the pocket. If it goes real easy, you had a high pressure round, or out of spec primer pocket dimensions. Also tear apart a new round from the same batch and see how the primer fit is on one that's unfired.

Winchester pulled the oversized primer pocket thing a few years back on a couple lots of Q3131 223 ammo. Stinkin' primers would pop out and fall down into the trigger group, tie it up real good. Easy stuff to spot as they crimped the heck out of those pockets trying to make it functional ammo. Might be federal's doing the same trick.

Tim James
January 19, 2009, 03:29 PM
Might be easier to measure the primer hole than find another one to pop in there, unless I can pull those out with a tool. I am making a parts order so I'll grab a bullet puller or something. I need some calipers too, I've been procrastinating on those for a long time.

If there is a small warp in the case rim, couldn't that mean that maybe the extractor was trying to yank it out before chamber pressure went down? Not necessarily trying to blame the gun, just brainstorming all ideas here.

January 19, 2009, 04:55 PM
Can you post a pic of a few spent cartridges? Maybe someone can see pressure indicators if there are any. Detailed close ups of the neck and base would be good.

January 19, 2009, 04:58 PM
go with the crimped primers for when it "needs to work"

Tim James
January 19, 2009, 05:03 PM
I'll try for some pictures. I am terrible at close-ups. Wish I had kept a normal round to compare.

January 19, 2009, 05:04 PM
If it was the black box XM193C, there were several reports over on ARFCOM of exactly that (blown primers) happening. Here ( are the results of a Google search.

Tim James
January 27, 2009, 10:05 AM
After thoroughly cleaning the chamber of the rifle, I had no problems with 40 rounds of Prvi Partisan surplus ammo. I even put 20 rounds of the XM193C through it with no issues, but that's not necessarily conclusive.

Still working the issue where something is scraping the case head slightly, leaving two marks on opposite sides of the brass. I'll see if I can get a picture. Is any of that normal, or should I have perfect looking brass after firing?

Ben Shepherd
January 27, 2009, 12:14 PM
The 2 marks on your case head should be there. One should be kind of a line, and the other should almost be a dot. The line mark is from your extractor, the dot mark is from the ejector plunger.

Tim James
January 27, 2009, 03:30 PM
Thanks. I was told they might decrease a bit as the rifle breaks in. All factory ammo so there "shouldn't" be any overpressure or brass issues.

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