Well ill put my money on the mag too. I wasn't sure on the load, I've never reloaded .223/5.56 just bought in bulk for the time being. If the OP has p-mags he should be able to do a pretty quick test.
"Combined synergy of a man and rifle is matchless, the steadiness of hand and acuity of vision, and finally the art of knowing how to make the rifle an extension of the body, all equate to the ultimate synthesis of man and machine."
"Death smiles at us all, all a man can do is smile back."