There are semi-auto rifles chambered in .223 with collapsible stocks. Depending on the length of the barrel, it could be concealed easily.
However, I have a suspicion that the shooter is using a standard bolt-action rifle that does not eject the spent brass. Unless the police are withholding the info, there's been only one instance where the spent brass has been recovered. However, I'm not sure that's been conclusively linked to the shooter.
Either the shooter is not ejecting the brass or he is recovering it. If he is shooting from these wooded areas, he would have to delay his escape and search for it in the dark. Or, he could be shooting from inside a vehicle.
If the weapon used can be easily concealed after the shooting, I think there may be a reasonable probability that the shooter may be staying in vicinity as a motel which would validate the theory that shooter merely waits for a white van getting ready to move.
I've been suspecting that the shooter has a support network that provides him a safe haven nearby. He can lay low for a few days and then depart without being challenged.
I think the white van hypothesis is a self-fulfilling prophecy. If the shooter ever used one, he most certainly is not doing so now. But, there are so many of them on the road, the probability of one randomly being in the immediate vicinity of a shooting is nearly 100%.