Since the story of the Mauser rifle being quickly disassembled on the rooftop then reassembled in the woods (with the scope backwards ) doesn't seem likely, that leaves the way open for other theories.

I was never convinced about the rooftop sniper shot because the trajectory and the injury looked wrong to me. To me it appeared initially that Charlie was shot from his right, with the visible ragged wound being the exit wound, not the entry wound.