My guess is in line with browningautorifles, that your rifle was built for use with the blank adaptors. This means the end of the barrel was threaded. A previous owner, probably not liking the look of the treaded barrel, cut it off. Probably the only solution to this is to have a barrel extension added to it that is the right exterior size with the bore oversized, so it doesn't interfere with the bullet exiting the barrel.