Today I experienced a jam in my AR.  This is rare, and I'm wondering if folks have a theory and the diagnosis of it, so it doesn't happen again.  

Gun: RRA NM A4 (20" BBL).  
Magazine: old 30rnd aluminum GI beater
Ammo: 62 gr ball - mild reloads
Position: Prone

Fired 29 rounds rapid fire in a simulated rattle-battle (which is to say, fire as fast as you can to hit a very fine target, from prone).  The last round did not feed, but instead the bolt lockback was activated by the magazine, and the last round was laying on TOP of the magazine lips.  I dropped the bolt and fired it, but this isn't something I've encountered before, and seemed weird.

My pet theory: this is a magazine issue.

I had an antique mag start doing that marked it bad its now what i use to hold a lower in a vise when i'm piddling around

Yup, the mag is toast.  You may be able to fix it with a Magpul follower and a new spring, but for the $$ you will spend, you can buy a Cprod or similar and get both with a new body...

I have a couple 20 rd c-product mags that did this last time to the range. I thought about tweaking on them but haven't done anything yet.

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