Need some diagnostic help on this situation.
Took this rig, in 6.5 CM, to the range the other day after it had been sitting in the safe for about a year. Only 350 rounds through it. All JP factory parts including the Silent Captured Spring standard for this rifle. Bobro one piece mount with a ZCO scope. Magpul magazines.
Prior to heading out I ran a couple of patches through the bore, took the bolt apart, wiped down the bolt, removed and wiped down the extractor lever and bolt face, and wiped down the BCG, all with M-Pro 7, re-assembled the bolt and lubed it and the BCG, light touch, with Mil-tec. I had cleaned all of this prior to putting it in the safe a year ago, so this was just in the spirit of good routine.
Had intended to just remind myself how the rig shoots, practice staying on it through recoil impulse, confirm zero, call it a day. Brought some Hornady 140gr ELD-M rounds, which routinely have shot .75 MOA or slightly better at 100 yards when I am competent behind the rig. Over the history of shooting this rifle no failures of any kind. Gas port was set for the Hornady a year ago, no changes prior to this outing.
So that's the set up.
Start shooting and get three good shots then one failure to extract. The spent cartridge is in the chamber. Not hard stuck, just wasn't extracted. Tap it out from the muzzle end with a cleaning rod and it drops right out, no hammering necessary. The shot was a center punch of a one inch paster at 100 yards, and it didn't feel like a hot load either how it sounded or how it recoiled.
Examine the bolt face, no debris in the extractor claw, bolt face is clean, no brass fragments, everything looks normal. Remove the bolt from the BCG, nothing in the BCG cavity that would impede the bolt cycling. Ran a pipe cleaner through the gas key on the BCG, clean. Swab out the chamber to be sure there isn't any debris there, all bueno.
Do it again and same result, this time four good rounds and one failure to extract.
Turn the gas port down to nearly closed and start the process of opening it a quarter turn at a time. No ejection at first, naturally, then as the port opens I finally get an ejection. Open it an additional 1/8th turn and shoot five rounds to confirm. All good. Changed to another Magpul mag just to be sure, once again five rounds all good.
Back on the rifle with a full mag (ten rounds), once again three good rounds and one failure to extract. Open the gas port another quarter turn. Once again three good rounds and one failure to extract. Open the gas port some more and get four good rounds then a failure. Kept doing that process until the brass was coming out of the ejection port dented, and still the occasional failure to extract.
I assume there is something obvious in my process that would account for the occasional failures to extract. Hope someone can contribute a suggestion or even a question that might help me through this. If there is something in my explanation of the set up that needs clarification, please let me know that.
Thanks for any suggestions.
