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.
G3Kurz
Thank you for the insights. To me the use of subsonic would be for the novelty in 5.56. I remember reading somewhere that 77gr subsonic in a 14.5" barreled AR had a muzzle energy of 155 ft/lbs. Standard high velocity .22lr I believe has 140 ft/lbs. I have used EBR 7.62x39 Jackhammer subsonic. The EBR website used to list the 7.62X39 subsonic as having 500 ft/lbs at the muzzle, but I no longer see the energy rating listed. I've shot it through an 11.5" AR upper with a AAC M47-2000 can. It was about as loud as my converted HK94 with my Gemtech Raptor can.
Tonight I was actually looking at the EBR website. There is an article at
Engel Ballistic Research entitled "The Use of Subsonic Ammunition in gas operated and other "Tactical" firearms". In the sixth paragraph it states that subsonic ammo should not be used in a fluted chambered firearm. What I got out of the article was that because of the low gas pressures the case won't seal consistently. If the case doesn't seal the chamber, the propellant gas can leak out of the back of the chamber, possibly causing a squib. Because of the inconsistency of chamber seal, function would be erratic. STR Arms is the nearest distributor to me. At there website there is a page with pricing for EBR ammo. Near the top of the page it states "Not for use in H&K firerams (SP) with fluted chambers".
There was a thread about some commercially available 100 gr subsonic that would have enough gas cycle a standard semi auto action. The link the guy gave listed the ammo at over $2.00 a round. That would be a rather expensive novelty. Especially if I have to build a "Subsonic Only" 53 with a non fluted chamber. I believe Jeff (ghilliebear2000) has had some luck with the low power 7.62x39 surplus ammo by reducing the spring pressure in the bolt locking lever in the carrier. That might be helpful if building a non fluted purpose built subsonic only 53. I think that a subsonic only 53 would be a project when I consider myself as an "advanced collector". Again, thanks for the input.
Scott
