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Joined:
Aug 2026
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CA, USA
Posted: 8/13/2026 4:24:52 PM EDT
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Hey guys, just built a new unsuppressed AR (build specs below) and I can only ever get one round of PMC 55g X-TAC to fire before either the action cycles but fails to strip and chamber a new round from the magazine or it gets jammed halfway through feeding and dents the neck of the case. After my troubleshooting (listed below), my current theory is that I somehow installed the gas block wrong and the rifle is undergassed to the point of short-stroking. The ejection seems weak, closer to the 4-5 o'clock arc. This is the first upper I've built, so it would make sense to me that I messed up the gas system. Less likely is a magazine presentation angle or seating height issue, but I feel like I've ruled that out by swapping the mag catch, mags, and entire lower for a known-good one. Please let me know what you think and what I can try to do to fix this. Planning on reinstalling the gas block and maybe seeing if Wojtek pinned the gas tube wrong, but I'm not really sure how to avoid repeating the same mistake. I would appreciate any tips or recommendations regarding properly installing the gas block and aligning the tube with the gas key on the BCG. Thanks. What I've Tried:
The Build:
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Shoot it with one round in the mag and see if the bolt locks open on empty. Do this with several mags. You have to really get the gas block out of alignment to block the hole in the barrel to cause low gas failures. May need to pull the gasblock and actually measure the gas port hole in the barrel to see if it is on the small side. |
Joined:
Aug 2026
Posts:
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CA, USA
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Gotcha, I noticed successful last-round bolt hold-open the few times I did actually reach the end of a mag and occasionally I could get through 2-3 rounds before another feed issue or empty chamber. I also tried holding the bottom-rear corner of the mag forward and backward to try to see if mag tilt was causing the feeding issues but it made no difference. I live pretty far away from the nearest non-shitty range and am pretty busy so I'm trying to amass as much feedback as I can before trying out my changes during live-fire. |
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Single mag loaded round, charge and fire, does the bolt lock back on the bolt catch, with the catch in front of the bolt? Rifle un loaded, pull back on charging handle all the way back. Does the charging handle tension remain the same from the center of the pull, all the way back to end of pull where the bolt face is behind the bolt catch about 1/4~3/8", or does is seem like the last inch'ish of charging handle pull to get the bolt just behind the bolt catch, is increasing in tension with something not right in the receiver extension tube parts? Note, if face of bolt is more than 3/8" behind bolt catch at end of pull, something is out of spec in the receiver extension tube parts. Pull a mag apart, remove the mag catch from the receiver, insert the mag body until it right against the bottom of the carrier, then look at the mag catch slot, against the mags side slot for the catch. The bottom of the mags side slot, should only be about 1/8" above the bottom of the receivers mag catch bottom of slot. Next, reinstall mag catch so the end of its threaded section if flush with end face of button, then insert mag body back in slot, allow catch to lock, then push mag release button, and pull upwards on the mag body to see how much higher you can get mag to go before its feed lips do bottom out on bottom of carrier. Bottom line, if you have a jam where the front of the round is in the chamber, but bolt is sitting/jamming the case by being on top of it (not behind the rim of case)., Then are having a bolt over feed problems. Also,please list what mags (manufacturer) are in play, since a lot of the cheap knock off mag will cause this problem. Hence always try to buy know good USGI mags, not some cheap knock off copies of such that can cause feed problems on their own. To add, if this feed over problem only happens as the rifle get hot, then need to do singe round load and fire test again in those conditions, since will bank that it a gas leak problem, that the heat is causing the metal to expand to cause short stroking problems then. If yes, then need to check gas block to barrel, and even carrier key to top of carrier for leakage problems. |
Posted By PlaymoreMinds:
'Twas not the <cough> sweet and innocent <cough> PlaymoreMinds...
<---skips away in frilly skirts to Candyland, leaving gutters and snorkels FAR behind.
'Twas not the <cough> sweet and innocent <cough> PlaymoreMinds...
<---skips away in frilly skirts to Candyland, leaving gutters and snorkels FAR behind.
Joined:
Aug 2026
Posts:
3
EE: 0% (0)
CA, USA
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Update: The gas block was slightly misaligned. Removed the handguard and noticed gas leakage residue on the gas block and noticed that the block was canted a degree or two. Removed and reinstalled the block (just eyeballed it) and held it in place while tightening the set screws (gas tube was at an angle and exerting a rotating force on the block). Took it out to the range and got through all 140 rounds with no stoppages. Thank you all for your help. |
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