Issue with HK91 bolt not going into battery in full auto
I am looking for some guidance from members with more experience than me in roller delayed blowback firearms in full auto. I've been running into an issue with my HK91 not functioning properly in full auto. I am using a registered sear installed in a burst pack that has been working flawlessly with my MP5. The trigger pack is mounted in a clipped and pinned G3 trigger housing purchased from HKparts. I replaced the HK91 bolt with a Rheinmetall full auto complete bolt assembly. The issue I've been dealing with is not being able to cycle more than 1-3 rounds in full auto or burst mode. When the stoppage occurs, a fresh round is in the chamber and the primer is very lightly struck:
I measured the bolt gap, and it is within spec (0.014")
What I suspect is occurring is that the bolt and carrier do not lock correctly before the hammer drops, leaving a large bolt gap and leading to the malfunction.
It even fired out of battery once, and I caught it on video:
(Don't worry, the rifle is fine. Oh yeah, the shooter too.)
I played with it after my last range trip and discovered that unless I do the "HK slap", the bolt and carrier do not lock properly. I suspect that this is normal, since it also occurs with the original semi-auto HK91 bolt assembly. However, in full auto, the bolt carrier can trip the sear release lever, and therefore drop the hammer, before the bolt and carrier lock:
Notice the dropped hammer and large bolt gap.
I suspect that this is what occurs during live firing.
I thought at first that a weak recoil spring was to blame, but I also discovered that the locking lever spring was much weaker on the 60s Rheinmetall bolt carrier than on the HK91 semiauto bolt assembly. In addition, the locking lever sticks out more:
Has anyone run into a similar issue? What is the relative contribution of the recoil spring and locking lever to the proper lock-up of the bolt and carrier? I'm going to change the locking lever and the locking lever spring as well as the recoil spring before my next range trip, but is there anything else you can think of?
It sounds to me like the release lever or carrier trip ramp may be incorrectly timed for your rifle. I think that the 9X series is supposed to all use the same release lever, but if your MP5 is a conversion, it may have had the carrier trip ramp or release lever ground out of factory spec to get the MP5 timed correctly.
The MP5 is a conversion. Not sure how I could tell if the trip ramp was modified. If I carefully ride the MP5 cocking handle forward while holding the trigger back in full auto, I think I am also able to get the hammer to drop before the bolt and carrier are fully locked, but contrary to the HK91, when this happens they immediately lock up. In the HK91, they don't get that extra push to lock up when I ride the handle and the release lever gets tripped.
Sear trip levers are the same regardless of caliber. The ejector levers have to be changed, of course. But light primer strikes could definitely be caused by the sear trip lever releasing the auto-sear too early. The hammer follows the bolt back instead of snapping into it. This doesn't explain how it would work on the MP5, but be sure the trip lever you're using is one meant for a friction type registered sear, and not a roller sear. The only other thing I would want to check is the recoil spring itself. If the bolt isn't making it all the way back, but enough to trip the sear, that can cause the same problem. I once had a 91 from a customer that had a bad recoil spring. It wasn't weak, it was like it was twisted up or something, and would drag all along the recoil spring rod. Swapping the stock completely fixed the problem. I can't think of anything else to check off the top of my head.
My recoil spring did not seem to be binding or particularly weak. I did end up swapping it for a new one. I haven't had a chance to try it out on the range yet, but the new one doesn't feel particularly stronger than the old one. I'm going to swap the locking lever and spring as well, just waiting for the parts.
The release lever is indeed the correct friction type.
Hopefully it's not a timing issue from a ground trip lever ramp on the MP5 like CAR suggested. How common would it be to have to modify the bolt carrier like that to properly time a gun? By the way, the MP5 was worked on by Terry Dyer.
If all else fails, I'll buy a tungsten filled carrier.
Last edited by hippocampus; 04-24-2019 at 04:04 AM.