[toddst]

. . . if it was a US sniper the round being used would almost certainly be 7.62 instead of 5.56. . .

I don't think so because the 7.62 produces a great deal more muzzle blast AND has excessive penetration for the ranges involved. No reason to increase being located by using a weapon larger and louder than required. There are other reasons to aviod the 7.62 but this is the primary one, given the circumstances of these shootings.

[Technogeeb]

I don't think so because the 7.62 produces a great deal more muzzle blast AND has excessive penetration for the ranges involved

I think the "ranges involved" is additional evidence for why the individual isn't a U.S. military sniper, not evidence for why a U.S. sniper would pick a different caliber. If he is a sniper, he is going to be "best trained" to use his preferred weapon (a 7.62), and he is likely to use it at the ranges in which he trained. He gets no advantage for going to a shorter range, and thus gains no advantage for choosing a lighter cartridge at those shorter ranges. In any case, as a military sniper (unlike a police sniper), there is no reason why he would be particularly concerned about over-penetration.

There is a bit of substance to your suggestion that he might be trying to avoid muzzle flash, but I think a well trained sniper would solve this problem with other means (flash suppressor, careful choice of cartridge loading and perhaps even custom hand-loading, etc.) instead of giving up the significant advantage of familiarity with his preferred weapon caliber. The simple fact is, this guy really isn't all that good, or is pretending to be a lot worse than he is. None of these shots are very impressive unless they're being performed under conditions of which we are unaware (such as from a moving vehicle). Considering the evidence, I can't really see why a U.S. military sniper would deliberately handicap himself if he were intending to perform the actions that we have seen this shooter commit.