Your description is a pretty common reaction of elk that have been duoble lunged. And it doesn't really matter if you shoot them with a 6mm NPT or a 338 NPT..
This fall I watched a cow immediately fold up when shot through both lungs and an exit with a 270 cal 150 NPT, while the cow I shot a couple days prior with a 150 NPT ran 40 yds after being shot twice through the lungs.
Casey
