The rifle that wouldn't lock open...

Chris Rhines · May 16, 2015

I have an AR upper built around one of the 18" Satern barrels from Doc's Gun Shop. I cannot, no matter what I try, get this rifle to lock back consistently on an empty magazine.

I've tried:
- Different bolt carrier groups (BCM full auto, JP stainless low mass, old-style JP Aluminum)
- Adjusting the SLR gas block from closed to wide open.
-Different gas blocks.
- Replacing the gas tube.
- Three different brands of 5.56 ammo (Atlanta Ammo 55gn VMax, Stillwood 69grn SMK, and IMI M193.)
-Different recoil spring setups (JP SCRS with the standard and light springs, carbine buffer and spring, carbine buffer with all the weights removed.)
- I put the problem upper on a different lower, in case there was something up with the bolt catch. No dice.
- The gas port measures out at 0.095". No burrs.

No matter what, this upper will fail to lock back empty 3 or 4 times in 10. It'll cycle just fine with the gas block at 9 clicks open or better, but it won't lock back.

I'm about at the end of my rope here. Is there anything else I could try?

Thanks,
Chris

drmitchgibson · May 16, 2015

Verified that the gas block is actually aligned? Sometimes you have to measure it. Some blocks seat flush at the shoulder and some need the gap that a handguard end cap provides. I would say it has to be gas flow or some mechanical obstruction.

Chris Rhines · May 16, 2015

Yup, the gas block is aligned. Forgot to mention that one.

Some additional research on the internets indicates that my gas port might be undersize. 0.093"-0.096" is the size range for a 20" barrel with a 13.2" gas system. Crane drills the gas ports on the Mk12 (18" barrel, 13.2" gas system) to 0.100"-0.105". JP apparently uses a 0.104" port, and John at WOA goes up to 0.108".

I think that next weekend, I'll run this gun down to Dad's shop and ream the gas port out to 0.105" or so. That may be all the more it needs. I really hope I can get this thing to run, because it is just sick accurate. Also, because my current rifle is showing all the telltale signs of barrel burnout. I guess it happens after five years and 25,000-odd rounds.