I dunno. M193 yaws and fragments even when fired through 1x9 or 1x7. It does break up better with a cannelure, which most civi factory 55gr loadings don't have. But thinly-jacketed varmint bullets (BlitzKing, V-Max, even Federal 50gr HP) are pretty explosive. The shooter's made hits at a variety of distances; it seems like most of the earlier shootings produced exit wounds, indicating a shorter range (head shot of woman on bench, coffee-cup sized exit hole on lawn-mower victim, blood splatter on mini-van at gas station). The shooting today was likely from longer range, judging by layout of school and fact that there was no exit wound.
5.45x39 is cheap, but not nearly as ubiquitous as .223. Also, only rifles chambered for it (that are readily available in this country) are Romanian AK clones. Not what I'd use for precision shooting.
The 5.45x39 is a very distinctive projectile, I think if they are saying .223 it probably is.
I would guess one of the very light projectiles from sierra or speer. I know one of those 2 will tumble if it hits bone.