I tried heat with mine and warped it. I hacked it off and replaced it with a cap from a Armor All pump bottle.
i still had to remove the old one and experimented with how I might do it if I had to do it again. I would cut it off with a hack saw, fine blade. Then use a sanding wheel on a Dremel to sand out that inner ring. That part should be clear anyhow an not over lapped if you refer to a real scope.
also, I used a clipped section of a lightsaber from a Darth Vader action figure to add the red tip on the scope.
IMHO, Spacebob's conversion is the best I have seen. Although if I had his skill, I would have altered the trigger. Also, some recent discoveries about the prop's side details have been discovered recently that would also need to be changed.