I'm guessing it is a standard .224 caliber load like the .223 Remington. Probably standard 55gr FMJ, which will undergo explosive fragmentation at relatively close ranges out of most rifles. This would make a lot of sense, since they seem to be saying it is a .22 caliber bullet and .223 is a ridiculously common version of that.
At the very least, the wound reports I'm seeming are completely consistent with the expected terminal effects of .223 at relative close range. A .223 can really chew up the insides of a person, even the FMJ.