Good point. I don't know of any oher gun that shoots this cartridge, certainly none on the American market. The police have been kind of tight-lipped lately, so who knows? It sounds like all they are recovering is bullet fragments, I know a .223 FMJ will often fragment into 3 pieces on impact, I'm not sure about the 5.45 x 39MM .
IT doesn't explain the ATF showing a bunch of AR-15's and HK .223's about a week ago saying "These could be the types of gun the sniper is using."
Well, you know how the ATF likes to demonize military-styled semiautomatics. However, I also saw a bolt action varmint rifle on that table. And this guy DOESN'T seem to be spitting out brass when he shoots, especially not in the last shooting, because it sounds like he was seen and didn't have time to look around for spent casings.