An exotic possibility is that they're using a Remington Accelerator, a 30-06 or .308 case with a sabot .223 bullet in it. High accuracy, and fantasic velocity. There'd be no rifling marks on the bullet, and the plastic sabot would blow away with the wind, and never be found.
I'm a bit unsure of what has been found so far. An intact bullet? A fired case? It seems the reporting is low on fact, and high on anti-gun hype.
It sure seems convenient that they have a fired case, considering how careful the shooters have been up to now. There are brass catchers for most brands of semi-autos, and anybody firing a bolt action would be unlikely to cycle the bolt, and lose the empty case, when he was getting one-shot kils.
There is something that smells about all of this.
Not really that exotic a possibility and actually a very good point. Shooting accelerators would have the cops looking for a smaller caliber weapon and no rifling marks would keep the perps in the clear if they were caught with their rifle.
Excellent point.
It is darned odd that the police were waving all those guns about, and that the anti-Ehrlich ads are picking up on this so quick . . . nah . . . I don't want to go there (snatching tinfoil helmet off head).