I'm not so sure. I am an AR-15 shooter, and a lot of guys get really into them. I have never seen a guy sit around shooting a .223 bolt action all day, but I have seen lots and lots of guys sitting around shooting scoped AR-15's all day.
Plus, my guess is that if they recovered any brass that you could tell by the marks if it was fired by a semi-auto or not.
The other thing I wonder is if this guy might be using some sort of silencer. The cops should check public garbage cans for 2-liter coke bottles filled with cotten.
Loud shots have been heard at more than one of the killings; quite a few, actually, I believe.
While using that kind of silencer would reduce the noise at least somewhat, I'd think that it would also reduce the range to where someone would have gotten a bead on the perp.
An example of the characteristics of such a .223 round can be found at handgunhunt.com:
"Over the long haul, the 40gr. Nosler Ballistic Tip with Blue Dot Powder is my hands down favorite for the 10" .223 Contender. This load is quite accurate, and as can be seen, only gives up a little velocity to the IMR-4198 Load. This load is so mild to shoot, while still offering excellent performance in the 10" barreled .223 Handguns it is hard to imagine. What has endeared me to the 40gr. Nosler Ballistic Tip / Blue Dot load is the mild report, almost nonexistent recoil, and the fact that it does not heat up the barrel as quickly as other loads do. This is extremely important to the Varmint Hunter sitting in the midst of a Prairie Dog Town, or for that matter, a heavily populated Flicker Tail Gopher patch."
Such a pistol is quite concealble and potentially "in-your-face" when compared to an AR-15 longgun and would be a good choice for the goblin sniper(s) in question........