The rifle would have been impounded by police.
It wasn't. There is video of him trying to surrender and the police ignore hm.
I don't think the rifle belonged to him since he claims not to have crossed state lines with it. I suspect it stayed in Wisconsin when he went back to IL.
Unraveling its evidentiary value is going to be a legal question since chain of custody was broken if he didn't take it back with him. That particular rifle might never make it into evidence...
Only if he took it with him home to Illinois. Or, if they found an owner that had it back from him in Wisconsin.