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Posted: 5/27/2025 10:42:36 PM EDT
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Alright , just finished an SBR build and am having failures going into battery (barely) fairly consistently. Built the rifle and shot it as a pistol before the alphabet boys gave me there blessing(100 rounds or so) without issue. Form one came back as well as approval for its suppressor , took it out (suppressed)and almost immediately started having failures to go into battery , hammer would fall and I’d get a light primer strike. Thinking it was the trigger and not realizing the bolt was barely out of battery I swapped it out with the same issue and noticed the bolt barely not in battery, could push it in by finger and rifle would fire/eject/lock back fine. Pulled off the suppressor with same results.Took it home , inspected/cleaned out the chamber (chamber was pretty dirty, but the whole thing was pretty nasty)checked extractor spring etc. Total I have maybe 300 rounds through it.I ordered an SA AGB, and a sico extra power spring to see if maybe the high back pressure with standard spring was causing the bolt to open too early fouling the chamber? Is that a thing? Ejection pattern was about 1:30-2:00 , shooting hand loads (small base size die) 77 grn smk’s over 23 grn tac .same loads run perfect in every other ar I have. Geissele 11.5” barrel Microbest chrome lined BCG H3 buffer/standard carbine spring |
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Since no one else has responded: ChatGPT: Light primer strikes = bolt not locked. Problem appeared after adding suppressor, but persists even without it now. Ejection pattern at 1:30–2:00, indicating overgassing. Chamber was dirty, but still having issues after cleaning. Loads are small base sized and work fine in other guns. 11.5" Geissele barrel, Microbest BCG, H3 buffer, standard carbine spring. 🔍 Diagnosis & Considerations 1. Overgassing + Timing Ejection at 1:30–2:00 = early unlocking, BCG cycling too fast. Suppressors add more backpressure, making this worse. If bolt is unlocking too early, it can: Not have enough dwell time to fully extract/clear fouling. Begin to chamber a new round before it's fully aligned/clean. Cause excessive fouling in chamber = bolt drag = OOB condition. ✅ Installing that SA AGB is a smart move. You'll be able to dial in the gas for suppressed/unsuppressed use and reduce backpressure issues. 2. Spring/Buffers H3 + standard spring = good suppressed setup generally, but if spring is worn or not up to spec, it could be too weak to counter suppressor gas. Extra power spring you're adding should help delay bolt movement and increase chamber pressure dwell. 3. Extractor / BCG Drag Microbest BCG is good quality, but check: Gas rings (should provide resistance — bolt should not fall under its own weight when extended). Cam pin smoothness (if it's rubbing on the upper or barrel extension lugs). Carrier key torque/seal. Any binding here would cause the bolt to fall just shy of battery. 4. Chamber Fit Even though you’ve small base-sized your brass and the same loads run fine elsewhere, check for: Carbon fouling ring at the chamber neck/shoulder junction. If suppressor-induced fouling hardened up (common after ~100 suppressed rounds), that can cause tight bolt lockup, especially with heavier bullets like 77gr SMKs. Try a chamber mop and bore solvent (not just a patch) to fully scrub. 5. Ammo Variables Even small variances in neck tension, OAL, or bullet profile can cause issues if the chamber is tighter than spec. Geissele barrels can be match-tight, so even though small base sizing helps, slight bullet ogive engagement can stop the bolt from fully locking. Try running some factory 5.56 (e.g., IMI, Black Hills, Mk262, etc.) and see if issue persists. 🔧 Action Plan Here's what I’d do in your situation: Deep Clean: Chamber brush + solvent (even a Flex-Hone if you suspect roughness). Install SA AGB and tune for ~3:30–4:00 ejection pattern. Swap in the extra power spring with your H3 buffer. Test with known-good factory ammo. Inspect bolt lugs and cam pin travel for drag/wear. 🧪 If Problems Persist… Swap lowers or uppers to isolate fitment/tolerance stack issues. Check headspace if you have GO/NO-GO gauges. Polish the chamber slightly if it's rough and confirmed clean. |
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After giving the rifle a thorough cleaning, to include cleaning the chamber with a chamber brush, then I would see if there are any parts that are rubbing out of the normal. Specifically try installing just the bolt carrier (No bolt, cam pin or firing pin assembly) into the upper receiver to see how it fits. The bolt carrier should move freely in/out of the upper receiver with no resistance. If you are getting resistance roughly the last 1" forward or first 1" rearward, then the gas key and gas tube are not mating correctly, which would be an indicator of either the gas block or gas tube is out of alignment, or the gas key is out of spec or bent. A simple check is to try and push the back 1" of the gas tube side to side in the charging handle track, it should have a little free float play side to side, and this side to side play should be the same amount side to side, if it isn't then you need to check alignment of the gas block as it could be off center, or if the gas tube passes through a barrel nut then the barrel nut could be out of alignment. Also as already mentioned, try some different ammunition, my recommendation is something that is new from a reputable American made manufacturer like Federal, Speer, Black Hills, Winchester, Hornady, Remington/UMC, etc. See if you get any different results, as it may be something as simple as ammunition related to your hand loads. CY6 Greg Sullivan "Sully" WeaponsArmorer SLR15 Rifles Defensive Edge Training (763) 712-0123 |
| Did you try a new BCG? I had a similar issue with an AR-10 I built and it ended up being the BCG slightly out of spec and rubbing on the magazine that was causing it to not fully go into battery. Before that I had tried everything, from new magazines and gas block to various buffer springs and weights. It was driving me crazy. |
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Just saw this today. You've got too many variables. Start simple. Strip it down to rifle only, factory ammo, and a few different mags. If it works, then add the suppressor. If it still works, it's your ammo. If the bare rifle isn't working with factory ammo, then I'd start checking bolt and gas system. |
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The bolt out of battery, Ditto and going with bolt (carrier) bounce, with chances are the gas tube is not indexed correctly to the carrier key. Hence carrier bounces back off face of battery extenion on load, but since gas tube to carrier key is binding, does not allow the carrier to come all they way back forward on it own again. So as stated, remove bolt and cam pin from carrier, then check to see if the two are aligned. If not, then gas tube needs to be tweaked over center of barrel to correctly index the gas tube. As for why with suppressor, and not without, would be that suppressor added more bore back pressure, has the bolt unlocking sooner with more speed, and your getting the faster cycle rate with the suppressor in play. So heavy buffer, or strong recoil spring, is the correct direction to slow the unlock back down to normal, with suppressor in play. To note, if just running 55gr ammo, only should need standard buffer without supressor in play. Running mk 262 type ammo with its slower burning powder that does increase gas port pressure since peak burn spike is closer to gas port, may require H1 buffer in play for correct unlock timing. Hence rig and ammo, may need H1 buffer un-supressed, and then H3 buffer with suppressed running the Mk262 type ammo. The other one that may be the problem is the reloads, so check finally loaded ammo with ammo gauge tool, since may not be trimming the cases enough, or could be over crimping the bullets to cause the cases to not be fully sized still, with it may not be bolt bounce, but just round is will slip into chamber correctly for those loads, and what is not allowing bolt to lock up to begin with for some rounds. The other one police the spent cases and take a good look at the spent primers, to make sure no signs of over pressure (spent primers pressure flowed all the way to the edge of the case primer pocket. Also, make sure your not long loading the bullets, and running into the bullets embedding at loading. A quick check here, remove the upper, pull the B/C, drop a loaded round in to the chamber, tap the back of rim with wooden dowel to make case shoulder it tight to chamber shoulder, then aim muzzle up, and live round should drop free of case. If not and you have to send a cleaning rod down the muzzle to be the live round out, start checking the bullet and case for where its binding in the chamber. Again, could be bullet crimping pressure too much to cause the case to be no longer fully sized, could be the case not trimmed short enough, or could be bullet long loaded too far out for the chamber in question. |
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.
| Thanks for the responses, checked what I can so far while waiting on parts. Gas tube/key are in good alignment, no resistance in any position with bolt removed. No evidence on carrier or lugs of excessive resistance / rubbing , several different mags were used during testing without change. Chamber is clean now. I will go buy some factory stuff to compare once rifle is assembled. FWIW ejection pattern was 1:30-2 without suppressor installed so I think the stiffer spring/AGB is warranted/necessary. |
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Originally Posted By North_42: Did you try a new BCG? I had a similar issue with an AR-10 I built and it ended up being the BCG slightly out of spec and rubbing on the magazine that was causing it to not fully go into battery. Before that I had tried everything, from new magazines and gas block to various buffer springs and weights. It was driving me crazy. Haven’t tried a different BCG, will bring a spare with me when I go out and test. |
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Another check is to see if the BCG is rubbing the magazine feedlips when you're resting the rifle on the magazine while shooting. You can simulate this by disassembling a magazine, insert it, push up on it while moving the BCG back n forth. Resistance should be easy enough to detect if there is any. You could paint the bottom of the carrier or top of the mag lips to visually see any interference, but it isn't required. |
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