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Posted: 9/16/2026 10:15:49 PM EDT
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My son and I built an AR15, and it's failing to lock back on empty. I've looked at it and don't see why this is occurring. Let me help us all skip a couple of causes that don't apply here - it's not a gassing issue or grit in the mechanism or warn parts, because it won't lock back when I manually pull the charging lever back on an empty magazine. The magazines all have firm springs and the followers come fully up on empty. It's a new build with only a few rounds through it, so the parts aren't worn. When I pull the BCG back all the way then slowly let it move forward, it still fails to lock back on an empty mag. With the lower open I verified that the bolt catch moves freely and all the way up. I removed and reinstalled the bolt catch and everything is clean and well lubed, etc. What could possibly be causing this? It's acting as if the bolt catch isn't built to extend high enough, or somehow the BCG was improperly machined. Neither is likely, so I'm puzzled about what to look at next. An earlier post mentioned the possibility of machining the lower a bit to allow the bolt catch to move higher up, but that's pretty radical so it seems as if there would be a far more likely problem, I just haven't figured it out. Does anyone have a tip for me? thx |
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| Do you have access to another upper and lower to be able to test what you have and see if it’s all uppers/lowers or just that combination? |
A closed mouth gathers no foot.
Make it "Idiot Proof", along comes a better idiot.
Official Site Nickname, given by NorCal - "Smack"
Make it "Idiot Proof", along comes a better idiot.
Official Site Nickname, given by NorCal - "Smack"
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Since this is your first post, welcome to the forums. Now if you could possible provide a little information as to what type of parts you built the exact rifle configuration with, it may help us diagnose what you are working with. Things like what type of stock and buffer tube, buffer type, action spring (buffer spring), folding stock, sling mounting plate at the back of the receiver, etc? Also remove the buffer and action spring (buffer spring), and give us an internal measurement of the depth of the buffer tube. CY6 Greg Sullivan "Sully" WeaponsArmorer SLR15 Rifles Defensive Edge Training (763) 712-0123 |
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| No, good point, I should have mentioned that initially - even pushing the bolt catch with the BCG back doesn't cause it to lock, so there seems to be a geometric problem. I wonder if dimensional drawings of mil-spec AR components is available anywhere? It's hard to believe that the bolt catch was manufactured too short but at least I'd be able to rule it out. |
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| Not sure if the buffer components would cause this, would they? Just exploring your train of thought. Since the BCG won't even lock back when the BCG is manually pulled back against the buffer and the bolt catch is manually pressed, I'm not sure I see how the buffer would interfere? But I'm considering everything now. |
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Originally Posted By Jeffpat: Yes, this is a plan, I'll do that today and see what happens. Attached File |
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| Yes, I'd have included all that in the OP but my son purchased the parts so I don't have access to the brand/model of the different components. The point about the buffer tube depth is that insufficient tube depth in a certain stock, or excessive buffer component length, would prevent the BCG from withdrawing far enoiugh for the step on the BCG to clear the bolt catch on the way back, so they couldn't catch on the way forward? I hadn't thought of that, I'll check that. |
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Originally Posted By Jeffpat: No, good point, I should have mentioned that initially - even pushing the bolt catch with the BCG back doesn't cause it to lock, so there seems to be a geometric problem. This is why I asked about what parts are used in the build, and about measuring the depth of the buffer tube. You should be able to manually lock the action reward by hand if the parts are in spec and assembled correctly. Having any parts that are out of spec can cause this, something as simple as the buffer tube internally being machined too short. Trying to use a rifle length action spring (buffer spring) in a carbine length buffer tube may be causing the issue. If you are trying to use an A5 buffer in a standard collapsible buffer tube could be the cause as well, as the A5 buffers are slightly longer and are supposed to be combined with an A5 buffer tube. If you are not sure of what parts you may have, can you remove the buffer and spring, place them along side of the lower receive and buffer tube, take a pic and post it here, as this may help us see what you are working with. CY6 Greg Sullivan "Sully" WeaponsArmorer SLR15 Rifles Defensive Edge Training (763) 712-0123 |
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Originally Posted By Jeffpat: Not sure if the buffer components would cause this, would they? Just exploring your train of thought. Since the BCG won't even lock back when the BCG is manually pulled back against the buffer and the bolt catch is manually pressed, I'm not sure I see how the buffer would interfere? But I'm considering everything now. |
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| I pulled the buffer out and found the tube is 7" deep from the inside end to the retention pin. The stock is a Luth AR, very highly variable, and I found if I extended the stock, the BCG locks back properly. That's an interesting limitation for a commercial stock, I may contact Luth to see if they have more info on why this is and how to work around it. Anyway, it's functioning, thanks to everyone for your ideas. |
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