If it was the 55 gr. battle packs, the boxes in them should have been the PMC Bronze.
It is rated at 3200 FPS, and should have given enough gas pressure to run properly.
http://www.sgammo.com/product/223-55...mj-ammo-223abp
The PMC -X are the 62 gr. "Green Tip".
Was there any over pressure signs on the expended cases ?
Or maybe you got an under powder charged batch causing low operating gas pressure.