Or you could get some nice kind engineer to turn you down a bit of scrap rifle barrel to 10.56mm diameter and angle the face of one end at 6 degrees. From that face end, the fine flat knurling starts at about 16mm and lasts for about 50mm. needs to be about 170mm long so you can cut to length and crown to match your old barrel. easy enough to do and Tempest /Hurricanes work quite well in .20 if you can stand the cost of the pellets Fools a lot of people if they borrow it!