I was between 300 and 400 rounds into a session today using a 6920 and Federal AE 55 gr. when I had a round that didn't go off.  When I ejected the round it looked like this:




The round has a light primer strike and I'm guessing that that's because it didn't fully chamber, out of battery.

I'm 99.9% sure that the round was not in that condition when I loaded it into the mag last night.
This is the one and only failure I had today out of 600 rounds.
Can anybody tell me what would cause this?  

Old magazine?

I have seen that type of case damage with worn-out and old mags with outa-spec feed-lips.

Quoted: Old magazine?

20 mags - 6 of 'em were on their first outing, with me anyways.  Got 'em from a friend fresh back from the sandbox, and I thought the feed lips on one of them looked suspect.  When it happened I didn't pay attention to which mag it was, just cleared it and went on.

The dents on it looks like it hit the feedramps, but how would that happen and then the round still make it into the chamber?  I've seen mangled rounds before, just not like this.

The dents actually look like the bolt got jammed on the round. They look like just the size of the chamber locking lugs.

So My presumption is this:

1) Round popped out of the mag.
2) Bullet missed the chamber and went low, with the case head high in the air.
3) Bolt closed on case "sideways" against the locking lugs of the chamber creating the two dents.
4) Bolt hit enough of the case head to force it mostly into the chamber
5) You pulled the trigger, but because the bolt was not locked, the hammer didn't hit the firing pin, but just impacted on the carrier or just partially on the pin.
6) Ejected.
7) Light primer strike due to the either weak firing pin strike or just the usual AR-ness.

Do basically, the round popped out of the mag and was not controlled feed into the chamber. Sounds like your mag feed lips are too "worn" (i.e. bent too far out) and unable to hold rounds securely in the mag under tension/gun movement.

So you can fix the mag, or just go buy a new for $10.

But I'm no expert...just an engineer.

That is indeed strange.

As No-Aim mentioned, marks like that are usually caused when a bullet FTF (fails / failure-to-feed) and a bolt jams on the top of the shell.

But when you said that the round apparently chambered (albiet, not fully) and that it had a light primer strike ... that simply is strange. To me at least.

I cannot imagine how the bullet could have chambered when it is clear from the dent on the round that the bolt hit it in the middle. No way that round could have been pushed in the chamber let alone have the extractor engage the shell and eject it.

I am in no way, putting your story down or giving you a hard time. I just honestly find it baffling how that could have happened.

My guess still is - a bad mag. Be sure to mark your mags (a,b, c) and keep a log when things like this happen. In that way, if this keeps on happening you ave a record and will know if you have a bad mag.

Good luck. Cheers.

Quoted: I just honestly find it baffling how that could have happened.

+1

Quoted: Quoted: I just honestly find it baffling how that could have happened. +1

I'm not. Back in my Navy days with CSAR training and FA Colt Commando's, I've seen some pretty crazy stuff where rounds end up getting stuck. Rounds getting stuck on top of the bolt/carrier, sandwiching the round between the top of bolt/carrier and gas tube space on the upper receiver. That was one jammed up mess that I'll never forget. Great stuff

Only other possibility is that it got dented by the feed ramp, you can try to match the dents by placing the shell by hand to see it they match the feed ram grooves, just a thought may be it chambered at a steep angle getting caught on the ramp. it looks that way to me anyway.