sound like there isn't enough threads exposed for the booster. Is your overall length correct? M53 is also longer than a MG42. You are also putting to much thought on the booster. There should not be any play in recoil nozzle. Once everything is correct, you screw on the booster until the recoil nozzle touches and loosen it a little. Too tight will cause issues when everything heats up while shooting. I acually had some very intresting chats with WW2 German MG42 gunner. He told me the first time he saw combat was against the Russians. They were already defending. The Russians attacked their line early one morning. He opened up his 42. Shortly after the first can the gun stopped firing. He noticed the booster lock had been damaged and the booster assy had unscrewed and blown off. He got out of his trench & had to crawl around to find that damb booster! He found it down range , quickly screwed it back on and and smashed the lock down! Just amagine this poor kid crawling around in the dark, under attack & trying to find these parts in the snow! I have accualy had a booster unscrew its' self while shooting a MG34. It was supprising how far it went and I found it still all together. The beauty of the 42 is it's simplicity.
Spell check is down and I'm too lazy to get the dictionary