"an AR can swap "uppers" in less then 5 seconds. Only the "lower" receiver is registerd. That is the aluminum housing for the trigger group. Everything else can be bought of the internet."Yep, you're right on. With a couple of uppers, some polishing compound, and a handful of different bullet weights, they'd be pissing in the wind for a looonnnngggg time trying to "trace" it.
It might not be a .223 at all.
Someone on FR mentioned that most .22 rounds will not fragment like the ones LE have recoverd when fired from a .223 into a target over 200 yards. A .22-250, or other higher velocity-higher powered CIVILIAN versions will.