Slow-mo capture: ARC "light strike/dead trigger" in 9mm AR blowback
I got out to the rapid fire range this week to test an ARC in a "stock configuration" FM 9mm AR blowback (5" VFM dual charge, ambi lower, A5 tube RMB). I was getting short bursts with dead trigger/light strikes. This setup had already worked just fine with a GMR S7 SS.
I didn't have all my troubleshooting tools/parts with me, but I did catch it on high speed - 3 shots in the FRT string - first shot normal, 2nd FRT normal reset/release, 3rd dead trigger. I did notice that the timing appeared slightly different between the 2nd and 3rd trigger pull/hammer release and the moment the bolt closes. The cam releases control of the trigger before the bolt is closed, which is expected, but if it's too early the hammer may hit the rear/bottom of the bolt under the FP rather than the FP itself.
Traditionally this type of failure in a 9mm blowback AR has been the result of bolt bounce, firing pin bounce, or a cam timing issue. My suspicion is that it's a cam timing issue, but next time I get out I'll do more testing.
The RMB has shown itself to be very effective at eliminating bolt bounce on high speed video.
My troubleshooting is going to be, in this order....
Tried already:
- change from newer FM firing pin spring (strong) to UXCELL firing pin spring - no change.
Next, in order until something works:
Install a BoreBuddy-type timing advance shim (actually bought a 25 pack on McMaster-Carr - Part # 99040A408). Installation video on BoreBuddy web site.
Titanium firing pin to help prevent firing pin bounce
Stronger Ace Hardware spring for firing pin to help prevent firing pin bounce
That should cover advancing the timing and preventing any FP bounce. From there I should be able to figure out which one is the problem.
Based on this post (https://www.reddit.com/r/AR9/comments/1si3mij/update\_on\_my\_psa\_gen\_4\_with\_arc\_fire/), it may very well just be timing.