The rifle that wouldn't lock open...

Chris Rhines

Active Fanatic
Joined
Feb 2, 2013
Messages
32
Location
Washington DC, United States
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
 
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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.
 
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.
 
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