I have limited experience with subsonic 5.56mm ammo in the HK53 and HK33K, more so with frangible training ammo - neither was good frankly. We tried various/all available locking pieces with poor results. There simply was insufficient recoil impulse from the cartridges to reliably operate the roller-delayed bolt. The plan was to develope a unique subsonic bolt broup (like the blank "blue" bolts) but that did not meet the user requirements for interchangeable ammo w/o weapon adjustment. The HK53 frangible ammo adapter had a so-called "self-adjusting" valve so that the weapon could be used with combat ammo too. It did not work out. True subsonic function in 5.56mm with most any type operating system has always been less than reliable. The best results I know of were with some special Black Hill's made 100+ grain ammo in the HK416. Folks are still working on it but 55-77 grains at 1000 fps is hardly impressive from a terminal ballistics stand point thus things like the 300 Black Out and Whisper rounds.Hey guys, my experience with 5.56X45 subsonic ammo has been with the AR platform. There isn't enough gas to cycle the action and keep a 77 gr bullet subsonic. Since the roller locked guns can be "tuned" with a different angled LP, I was wondering about using the subsonic in a 53 or a 33K clone with a can and a 70 degree locking piece. I'd think that with such a high angled LP, one or the other gun might function with the extra energy a can would add.
Sorry if this has already been covered. I have no idea how to specify this application in a search. I was just wondering if anyone had tried it. Please let me know what you think.