I agree with every thing 64Springer said. I cut a CVA Kentucky rifle down to 28 inches. I did every thing Springer suggested but I had some 3/4 inch brass balls that had a 1/4-20 threaded hole in them. (I don’t remember from where but it was off the web) I put a bolt in one and cut off the head. I chucked it in a hand drill and used it to smooth the crown surface. I rotated the drill at all different angles. It took sometime, maybe an hour, but it worked. I got the idea off Youtube. I’ve also used a glass ball that belonged on top of a lamp shade. I got that at Lowe’s. Here are pictures of an original blanket gun I’ve been thinking about copying. It may be similar to what you are going to do. Good luck and share pictures.