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Posted: 5/30/2015 10:59:17 AM EDT
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Specs:
6920 lower w/Wolff std buffer spring & H3 buffer, BCM LW 16" middy upper. This has been 100% with everything I've fed it from M193 to weak PMC Bronze. This morning tried a new chromed bcg, and while feeding/extraction were 100%, the bolt wouldn't lock back after the last shot most of the time using my std Wolf Gold. Switched to a std carbine buffer and finally started getting fairly reliable lockback but it's still not 100%. So, am I looking at a simple wear-in problem that's eventually self-correcting or is there something more serious going on here that I'm missing (I examined the gas key and I don't see any obvious leakage)? Tomac Update: Tried another new bcg from the same mfgr in an otherwise identical AR and got the same questionable lockback reliability. Checked the gas rings on the chromed bolt and they were visibly undersize. Replaced them w/spares and ran to the range this morning. My AR is now shooting the weak PMC Bronze w/the nearly 100% reliability of Wolf Gold was exhibiting yesterday; function is 100% and locks back most of the time. Function & lockback w/Wolf Gold is now 100%. Since both bcg's exhibited identical behavior right out of the box, it looks to me like the mfgr got a bad batch of gas rings so I will replace them all accordingly. |
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Dry fit the Carrier with key into the upper by hand, and see if there is any binding of the sides of the key to the upper. Also, new bolt will need to mate with the barrel extension. So on that note, make sure to run the B/C slightly wetter inside and out for the first few hundred rounds.
As for the key, need to pressurize it to see if it leaking between the key to top of carrier. Put some CLP around the entire base of the key, hold the bolt into the carrier, and pressurize the carrier gas chamber with about 100lbs of air pressure through the front of the key. As for H-3 and M-193, too much buffer mass to begin with. The H-3 is reserved to help 69 gr+ ammo to cycle in the rig (slow the unlock back down), since the heaver ammo uses slower burning powder, which pushes the peak dwell of the powder burn closer to the gas port, which increased the pressure through the gas port to the action. With standard M-855 downward, all that should be needed is just the standard weight buffer instead. |
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Quoted:
Dry fit the Carrier with key into the upper by hand, and see if there is any binding of the sides of the key to the upper. Also, new bolt will need to mate with the barrel extension. So on that note, make sure to run the B/C slightly wetter inside and out for the first few hundred rounds. As for the key, need to pressurize it to see if it leaking between the key to top of carrier. Put some CLP around the entire base of the key, hold the bolt into the carrier, and pressurize the carrier gas chamber with about 100lbs of air pressure through the front of the key. As for H-3 and M-193, too much buffer mass to begin with. The H-3 is reserved to help 69 gr+ ammo to cycle in the rig (slow the unlock back down), since the heaver ammo uses slower burning powder, which pushes the peak dwell of the powder burn closer to the gas port, which increased the pressure through the gas port to the action. With standard M-855 downward, all that should be needed is just the standard weight buffer instead. After 200rds, dry fit is now smooth (was a little rough at first, even w/extra lube). Did the pressure test as best I could (canned air, don't own a compressor) and while the bolt unlocked from the pressure, there were no signs of leaks. H3 & M193 runs 100% in both my BCM LW 16" middys w/std carbine springs, never a problem of any kind, even w/weak PMC Bronze. Switching to std carbine buffer did significantly improve bolt lockback. Tomac |
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