I hadn't really thought of it, but the shorter .300 case might allow the action to close enough on a 5.56 chambered rifle to get the firing pin to hit the primer. I bet a free floating .30 caliber bullet exploding in a 5.56 chamber makes for all sorts of unpleasantness. On a related note, I had a buddy who inadvertently loaded a .223 round into his AK 47 mag. Yes, it fired the round, and gave a college try at ejecting the exploded case. I love Mikhail Kalashnikov.