I'm not sure if this is a bolt, or may issue. I doubt it's ammo. Since I wasn't sure where to put it, I decided to post it in general AR.

Specs:
Lake City M193 5.56mm NATO . Two rounds in question have a '99 and '00 stamp
Rifle: ArmaLite AR-180B
Magazine: Stock Colt marked AR-15 20 round mag.


Last two times I used this mag, on the last round, the round ended up I think, between the feed lips and the bolt. It closed on an empty chamber. The round never entered the chamber.

What it did was I've got a scratch from one of the lugs that runs from the rim area all the way up to the shoulder . Right before the shoulder of the brass, there's a pretty good sized dent. There's also a small scratch and dent on the neck of each round up to the bullet itself on the opposite side from the big dent. The two times it did it  (I thought the first one might be a fluke, so I reloaded the mag with three rounds to see), there was no indication of a problem until I heard "click" and then no "bang". Waited to see if I had a hangfire, and when I dropped the mag, out fell the round. I have never before had a problem with this magazine.  It's one of my standard "range" mags.

Anybody have any thoughts?

Weak mag spring is where I would start. Try another "known" mag before you go any further, start at the beginning first. Sound as if the bolt is beating the mag spring and not really grabbing the last round well.

Quoted: Weak mag spring is where I would start. Try another "known" mag before you go any further, start at the beginning first. Sound as if the bolt is beating the mag spring and not really grabbing the last round well.

Ditto...I was thinking the same thing.