Failing Function Test

[GriffonSec]

Intermittently ............

Function test - Pull the trigger and hold to the rear. Cycle the charging handle to re-cock the hammer.

Release the trigger to reset the hammer.

The hammer will slip off the trigger engagement surface and fall forward. It does not properly reset. It will not do this every time, but frequently.

It will do this with the upper on the lower, and without the upper in place.

The fire control group has functioned properly for 3 to 4 thousand rounds. There does not appear to be abnormal wear. We have replaced all fire control springs, and it continues to fail. We replaced the hammer and it continues to fail. We have used an auto selector (factory supplied pre-ban) and an AR-15 safety. All NFA rules followed.

We replaced the entire fire control group with a new Daniel Defense fire control group and it began to fail the function test after 50 to 60 rounds.

Any ideas ??

[Paulo_Santos]

If you have access to a complete trigger and safety from a know good working lower, I'd swap it out onto that lower and see if it works. If the known good trigger and safety doesn't work, then I'd start checking the lower receiver for some problems.

[5pins]

It sounds to me like your lower is out of spec. I can’t think of any other way two different fire control sets could fail like that. What brand of lower is it?

[GriffonSec]

Lower is a S&W MP15. We posted this over on M4C and got the same suggestions. The old trigger set was pulled from a working lower, now it does not, back in it's original host. I picked up another DD LPK and installed the trigger into the new build. We mic'ed the pins on an older S&W, the new S&W, and the old SGW the other trigger set was pulled from. Measured top to bottom, and left side/right side, there were no variances, the new S&W and old SGW were identical (1.05), the older S&W was a thousandth closer together (1.04). If my math is right, and CAD measurements I've seen are correct, a .152 pin added to the .894 measurement of the pin holes on center comes out to 1.046, which leads me to believe they're all within spec. The new trigger set in the new build still passes functions test fine after another 125 rounds of shooting yesterday, so I'm at a loss as to the cause. The only commonality between these was both triggers were in the new S&W lower at some point.

It gives me an excuse to pull the SSA trigger from my SBR and try the new SD-C trigger. I'll just drop the SSA into the SGW rifle, shame though, the original trigger it had was wonderful, and my favorite. There's got to be a reason both trigger sets failed in that one lower, and managed to do something to my good set to make it unserviceable.

[Doubletap13]

So the specs on the trigger parts were all sound, what about the specs on the lower itself?

[GriffonSec]

So the specs on the trigger parts were all sound, what about the specs on the lower itself?

Honestly, the trigger area was all we checked, as that would have been the area of concern regarding any misalignement with the intermittent Pass/Fail on the functions test. Posted measurements were of the lower with the trigger assembly installed.