Ok I'm mystified as to what can cause a double feed. My wife was having them towards the end of the day today. Here is the scenario

Colt 6920 (the bolt is not colt, but the carrier and everything else is)
Total shots fired: 146 rounds
Magazines: US GI 30 and 20 round types
Shooting positions used: Sitting, prone (both with 20 round magazines) standing (with 30 round magazines)(magazines were never used as a monopod)
Federal XM193...no problems all day
Georgia Arms 55gr FMJ - that's when the double feed problems occurred.

Now I have experienced the same problem when using GA in the past and am inferring that its the GA ammo. My question though is why? What would be wrong with the ammo that would be causing this.