dented case! what the ****? Anybody seen this before? Should I be concerned?

[REB]

So the next question is what mags are known not to cause these dents and has anyone modified the AA mags to prevent these dents?

[LRRPF52]

Those dents come from the barrel extension teeth. One will be larger, and the other smaller, indicating what side of feed presentation it came from in the mag.

It usually is when someone has too fast of a cyclic rate, or not enough gas, and there is a bolt over base malfunction.

If it's happening without a malf, and is chambering, it appears that the cartridge is impacting the teeth of the extension with enough force to cause these dents.

I personally try to set a gun up for optimal gas, with as low of a cyclic rate as possible. I also de-edge my feed ramps, blend and polish them on any AR15 I'm doing.

It happens on 5.56 guns too occasionally, people just don't notice. I saw it yesterday evening at the range.