We Marines appreciate your vote of confidence but even we can miss a critical target if a weapons isn’t properly zeroed: remember that Oswald supposedly to a shot at a General Walker earlier and missed him at close range with that same rifle.
The Walker shooting was at closer ranger, but he didn’t completely miss. His bullet hit the window frame, and got deflected, so Walker was only hit with fragments. That was only the first shot though, and we know the first shot at Dallas missed also. If Oswald had time for a second shot at Walker, Walker would probably be dead too.