Today I finally took out my 10.3" build and I experienced some problems while shooting it. The build is as follows.
10.3 DD barrel with .078" gas port
BCM mod 0 gunfighter comp
Micro MOA Govnah gas block with .070"/.078" ports
BCM KMR 9
BCM BCG
BCM upper
H3 buffer
USGI buffer spring
I had no issues what so ever using the .078" port, but I had a few problems when I switched to the .070" port.
with federal xm193 I had one bolt over base malfunction and one true double feed, trying to feed two live rounds.
with pmc 223 and tula 223 I had at least 5 short strokes and failure to lock back on the last round.
I was a bit shocked I had these problems especially with the xm193 and all the while I thought I had an H2 buffer in there as opposed to the H3. Do you think that extra weight would be enough to cause the problems I had with the xm193? When I arrived home I took off the KMR to verify that the ports were in correct order and they were, I even double checked the port size. Now that I think about it, it might be possible that the bolt over base malfunction was with the large gas port but I am not 100% sure, maybe, but I doubt it. This was also with at least 7 different mags
I am assuming that you’re using a standard length carbine buffer tube. I would try the H2 again, or even a H buffer to see if that cycles better. I run an H in the same setup and it works flawlessly now. Only if I run crappy Tula or something (not ever) I will get the short stroking problems, and no lock back on last round.
IIRC, Black River Tactical says his gas port inserts act as though they are slightly larger than size (.073 insert acts like a .070 gas port). I wonder if using a MircoMOA with a .070 gas port has a similar effect. Also, I would try an H2 buffer. If the symptoms continue, then try an H.
You have an interesting combination of circumstances going on here. I can’t help but wonder if they’re all related of if you could have different problems causing different malfunctions. Replies in red.
No the double feed and bolt over base were with separate mags, actually all malfunctions were with separate mags. The short strokes were with either USGI D&H or CPD mags, but the double feed and bolt over base were with PMAGs. Just out of curiosity how does crane get a .071" port if there is no .070" drill bit size? The next size up from .070" is .073".
I’m sorry if my number was off. However, it doesn’t change the problem the OP is having. The combination of the small gas port with heavy buffer and weak ammo is likely causing the short stroking and failure to lock back after the last round.