Debugging a JP adjustable gas block

Jul 25, 2013
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#1
I have a JP LRP-07 in 6.5 Creedmoor with an adjustable gas block. The block has an adjustment screw that enters from the right side of the block looking at it from the breech end of the rifle; and another screw that approaches the adjustment screw from the top of the block, which is used to lock the adjustment screw in place. The rifle has been a hammer and operating well, spent cartridges ejecting at 4-5 o'clock and bolt locking back on last round. Adjustment screw in the gas block was fixed in place and the locking screw hadn't moved.

Until a recent weekend outing when after 20 rounds or so the rifle suddenly would not lock back on last round. Fired several more rounds, one at a time, and still no lock back. BCG and bolt both properly lubricated. Same magazine used throughout. Ammunition during the entire experience was Hornady ELD-M 140 gr commercial rounds.

I checked the adjustment screw, thinking it had moved and closed down the gas port. Not so. In fact it spun loosely in its threads and I could have backed it out completely. I did back it out some, no change on lock back; backed it out some more, still turning loosely, no change. Finally the adjustment screw is barely engaging the threads that lead into the block. Still no lock back. I tried screwing the adjustment screw all the way in, to start the process of finding appropriate gas pressure from the beginning. Adjustment screw feels loose in the socket. The threads look worn, possible the locking screw has damaged them. Backed the adjustment screw out a quarter turn at a time, fired a round, still no lock back.

The block itself seems to be in the same position it has been since the rifle arrived from the factory and operated well. No apparent shift fore or aft and still seems perfectly vertical on the barrel. The spent brass has minor swipe marks, but no different from when the rifle was operating correctly.

Interested in any thoughts here. JP is sending me two new screws, to test that avenue, but given the port seems to be open and still no lock back I am thinking there must be another root cause at work here.
 
Last edited:
Feb 16, 2017
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#5
another thing to check is that it is overgassed. Not locking back can be from both under and overgassed, essentially the BCG is moving to fast for the bolt catch to grab, I would screw it all the way in, then back out one full turn or 5 detents if you have that, that should short stroke and then fine tune out from there. Are you using an HP bolt?
 
Jul 25, 2013
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#6
Yes, it is an HP bolt. This is a standard JP rig, I haven't modified it. I thought about it being overgassed but at this point can't turn the adjustment screw all the way in, it just floats in the block. JP has sent me a new adjustment screw and a locking screw, so I am going to clean the block and start from scratch with the gas port fully closed and start opening it a quarter turn at a time and see what happens. The rifle has about 500 rounds through it, so would be surprised if the block is really so fouled that it is causing short stroking, but that said this is the first time this situation has happened so I don't have any experience to judge it with.
 
Oct 2, 2005
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DFW Tx
#8
Dx that block and go SLR. I did that a while back. Used to set the JP screw and locktite it, but it would always back out at the wore possible moment