Even the 12 ga gas tube did not work when I did not have a tight seal to the gas block. Once I made a very tight, no leakage at all seal, the gun cycled great and only after several hundred shots did it start to egg out the butt stock attachment holes. A rubber buffer stopped that and just to see what would happen, I drilled a gas relief hole about 1/2 in ahead of the rear most travel of the gas piston. I have build 3 MG47s now and all of them didn't work until I made a no leakage gas tube to gas block seal and a very close tolerance gas piston to gas tube fit. CNTRAILRIDER and the other builders that have successfully got one to cycle have found the same thing. Granted, the 12ga diameter gas tube and piston was over kill, but once I got a no leakage tube to block fit and a no more than .005 thousands clearance piston to tube fit with a standard diameter piston and a tube that did not relieve the gas pressure until just in front of the rear most travel did I get reliable cycling. The ES and BP versions both use a standard AK piston but a very tight tube to gas block seal and a tight clearance piston to tube I.D. fit.