My bad, I ment a 14.5×114mm, and 20MM rounds aren't common and are usually specialty rounds, thus you wont really find anything about them, there used to be conversion kits for PTRS's to 20MM but it died out for whatever reason. And I should mention I have first hand experience with the PTRS, I actually used one in my service with the U.S Army, didn't get to use it often but I still had to maintain it. The muzzle flash appearing at the back end is a problem with the gun itself, the barrel to be specific. In my service days I had the same problem until I switched to a heavier heat sinked barrel.
