Generally speaking 20” barrels are some of the smoothest operating 6.5 Grendel with rifle length gas system. The 20” 6.5 Grendel barrel should work with the carbine buffer tube, carbine spring plus carbine buffer. You stated the rifle functioned for 700 hundred rounds. Some times we see extractor failure from being over gassed but having a short chamber can increase pressure which can shorten the life of the extractor and/or bolt.
I think you really need to figure out which bolt your rifle is chambered to use. If you determine the bolt is in fact use a 6.5 Grendel .136 + or - .001 then you know you have the correct extractor. If your barrel was chambered for an X39 bolt you might have the wrong extractor which is causing your problems. An improperly machined extractor which is out of spec can mark up the rim of the cartridge case and shorten the life of the extractor. If it were my rifle I’d do my best to determine whether the bolt has a .136 bolt face depth or was chambered to use a 7.62X39 bolt with .125 bolt face depth.
If you have a short chamber the round may chamber but might be hard to extract. An easy way to check the chamber is to remove the bolt carrier group. I’d give the barrel a good cleaning paying special attention to the chamber. You can try to look into the chamber and try to see that the chamber has a smooth clean surface. A rough chamber can inhibit extraction which is hard on extractors. With a clean chamber drop a factory cartridge into the chamber. Raise the muzzle and the cartridge should fall out. If you have to tap the upper or use a cleaning rod to remove the cartridge you may have a short chamber or a rough chamber.
If the cartridge falls out of the chamber then I would put an O ring on the extractor spring. The O ring adds strength to the extractor spring to help the extractor hold the rim of the case. I’d also suggest you load a single round into the chamber without a magazine in the well. The rifle should fire and extract the cartridge and notice where the cartridge lands. With a rifle length gas system the cartridge should land from 3-5 O Clock. If the spent brass is ejected to 1-2 O Clock this could indicate the rifle is over gassed. It is possible the gas port is not drilled to the proper diameter.
If the rifle will fire but not eject the chambered round you have an extractor issue, chamber issue or gas issue. It is possible the gas block has come loose or your loosing gas pressure where there is not enough gas to cycle the bolt. Inspect the case that did not extract and see if the extractor has left marks on the brass.
If the rifle cleanly fires and ejects to 3-5 O Clock try loading one cartridge in the chamber and leave an empty mag in the well. Fire the rifle and the brass should be ejected and the bolt should lock back on an empty mag. If the bolt does not lock back you may have a gas issue or a faulty mag.
If the bolt locks back on an empty mag then put three cartridges in a mag and chamber one round. Fire one round and see if cartridge fires, ejects and feeds the next round. If the rifle has a failure to extract take a picture of the malfunction. It could be a problem with the magazine, problem with the extractor or a bolt momentum/gas problem. We’re basically trying to isolate what is causing a malfunction.
Give my suggestions some consideration and see if you can isolate the cause of the malfunctions. It would help if you take pictures of the barrel and any barrel markings as to manufacturer or chamber type. With pictures we might be able to identify the barrel or see a problem with the gas block. Pictures of your bolt and extractor plus spent cases might be useful.
I think you really need to figure out which bolt your rifle is chambered to use. If you determine the bolt is in fact use a 6.5 Grendel .136 + or - .001 then you know you have the correct extractor. If your barrel was chambered for an X39 bolt you might have the wrong extractor which is causing your problems. An improperly machined extractor which is out of spec can mark up the rim of the cartridge case and shorten the life of the extractor. If it were my rifle I’d do my best to determine whether the bolt has a .136 bolt face depth or was chambered to use a 7.62X39 bolt with .125 bolt face depth.
If you have a short chamber the round may chamber but might be hard to extract. An easy way to check the chamber is to remove the bolt carrier group. I’d give the barrel a good cleaning paying special attention to the chamber. You can try to look into the chamber and try to see that the chamber has a smooth clean surface. A rough chamber can inhibit extraction which is hard on extractors. With a clean chamber drop a factory cartridge into the chamber. Raise the muzzle and the cartridge should fall out. If you have to tap the upper or use a cleaning rod to remove the cartridge you may have a short chamber or a rough chamber.
If the cartridge falls out of the chamber then I would put an O ring on the extractor spring. The O ring adds strength to the extractor spring to help the extractor hold the rim of the case. I’d also suggest you load a single round into the chamber without a magazine in the well. The rifle should fire and extract the cartridge and notice where the cartridge lands. With a rifle length gas system the cartridge should land from 3-5 O Clock. If the spent brass is ejected to 1-2 O Clock this could indicate the rifle is over gassed. It is possible the gas port is not drilled to the proper diameter.
If the rifle will fire but not eject the chambered round you have an extractor issue, chamber issue or gas issue. It is possible the gas block has come loose or your loosing gas pressure where there is not enough gas to cycle the bolt. Inspect the case that did not extract and see if the extractor has left marks on the brass.
If the rifle cleanly fires and ejects to 3-5 O Clock try loading one cartridge in the chamber and leave an empty mag in the well. Fire the rifle and the brass should be ejected and the bolt should lock back on an empty mag. If the bolt does not lock back you may have a gas issue or a faulty mag.
If the bolt locks back on an empty mag then put three cartridges in a mag and chamber one round. Fire one round and see if cartridge fires, ejects and feeds the next round. If the rifle has a failure to extract take a picture of the malfunction. It could be a problem with the magazine, problem with the extractor or a bolt momentum/gas problem. We’re basically trying to isolate what is causing a malfunction.
Give my suggestions some consideration and see if you can isolate the cause of the malfunctions. It would help if you take pictures of the barrel and any barrel markings as to manufacturer or chamber type. With pictures we might be able to identify the barrel or see a problem with the gas block. Pictures of your bolt and extractor plus spent cases might be useful.