You might have a good gunsmith thread a false muzzle to go on the end, not even close to rocket science. The thread line will not show or barely show. You might contact the gunsmith to see if he would do this for you for free.
I have had false muzzles put on contender pistol barrels that were 14" long and I needed to bring them to legal length for rifle use, they just have to be epoxied on to be legal.
