View Full Version : Bolt not locking on last round
I have browsed a couple write ups on this issue after scouring the net. Need help going over the best method or order of elimination for diagnosing the issue. As the title says my AR starting having issues not locking the bolt back after firing the last round. Probably in total 140 rounds or so with this setup. Didn't notice this issue until 50 or so rounds ago.
Set up
Upper: BCM BFH Middy ELW 14.5" w/ BCM upper receiver group
Lower: Noveske Gen2 lower with G&R lower parts kits, LMT buffer tube, H buffer, SSA trigger
Ammo: Federal XM193, All brass ejects at 3o'clockish
1) Is it the magazine? I tried 5 different pmags, different generations and colors. All same issue with bolt not locking back on last round.
If it is not the mag what should I look to next?
Bolt catch/spring, buffer, gas key? What should I try next to be efficient in fixing this?
First check is mags. You seem to have covered that. Next check is are you losing gas. Is the gas key staked? (Since it's a BCM gun I would think so) Are the gas rings good? If the gun was working before these are your normal failure points. Check the bolt catch to see if it might have broken...not normally an issue but certainly possible. Last is check the buffer spring. Is it worn (too short)? Should not be buffer unless you've changed it out before the issue started.
How many rounds through the gun?
Ammo could be a possibility, but again if it worked with this ammo in the past it is probably not the issue at this point unless you got a bad lot.
1) Lock an empty mag in the mag well. Rack the action completely to the rear. BCG should lock open. If not, it's usually the bolt catch. It could also be the mag. Try a couple of different mags
2) If the rifle passes the above test, load a magazine with a single round. Lock the mag in the mag well, load the rifle and fire it. The mag should lock back. If not it's usually due to short stroking. It could also be an out of spec bolt catch or magazine. Try a couple of different magazines a couple of different times
Bolt catch operates normally with empty mag. I tried loading 1 round in each mag. Same result with bolt not locking back after fired round.
1) Lock an empty mag in the mag well. Rack the action completely to the rear. BCG should lock open. If not, it's usually the bolt catch. It could also be the mag. Try a couple of different mags
2) If the rifle passes the above test, load a magazine with a single round. Lock the mag in the mag well, load the rifle and fire it. The mag should lock back. If not it's usually due to short stroking. It could also be an out of spec bolt catch or magazine. Try a couple of different magazines a couple of different times
Bolt catch operates normally(locks back) with empty mag. I tried loading 1 round in each mag. Same result with bolt not locking back after fired round.
The other static check is total travel.
Pull the charging handle all the way back and hold.
Verify the front edge of the bolt lines up with the rear edge of the ejection port.
Assuming that checks out,
Was this a factory assembled upper? What about the BCG?
The other static check is total travel.
Pull the charging handle all the way back and hold.
Verify the front edge of the bolt lines up with the rear edge of the ejection port.
Assuming that checks out,
Was this a factory assembled upper? What about the BCG?
Yes a complete BCM upper receiver group assembled upper with BCG. The bolt does not appear to be completely flush with the ejection port but neither does my 6920 when i checked on there. Maybe a few millimeters from flush. After manipulating my bolt catch it seems "sticky". Surprised I didnt notice sooner. Could it be as simple as a sticky bolt catch?
I had a very similar problem, ended up being the bolt catch spring was weak. It would rarely lock back on an empty mag, but it would lock back periodically in the middle of a magazine. I was blaming it on magazines at first, then noticed the bolt catch seemed off. Replaced the spring, no issues since.
Try a standard carbine buffer and see if that does the trick. A buddy of mine just got the same upper as yours, but also a BCM lower, his came with a standard carbine buffer.
I recently built a lower and the magazine catch was dragging in the slot in the lower. If not the spring that could be an issue too.
Take your lower off of your upper, then insert a magazine and check the engagement between the bolt catch and the follower.
Please post a photo here for reference, if possible. I've had issues with magazine followers "moving" enough to not catch. You want as much engagement as possible, without interfering with function.
Example (not my picture):
http://www.hunt101.com/data/500/ar111.jpg
I'm having the same problem with my latest build. I pull the charging handle back, engage the bolt catch by hand and it holds open until the rifle gets "jarred". This happens if I give the rifle a good shake or sometimes when closing the charging handle. This is not an issue with any of the other AR's I built or the factory built HBAR. What are the correct measurements from the bolt catch to the lower receiver when at rest and when deployed? I'm thinking the catch may be out of spec. It was a DPMS lower kit from Brownells.
Kansaswoodguy
12-01-16, 04:18
Sounds under gassed make sure the gas block is lined up with gas port in barrel make sure the gas port is clear sometimes when the port is drilled a little bur is left behind running a drill bit in the port hole with just your fingers should clear this. Check that the gas key is tight and properly staked.
Extra info on my problem, I haven't even been to the range to test fire it yet. Found the problem while doing a routine function check. And the measurement I was looking for was from the top of the catch to the top of the receiver.
14.5" and mid-length gas is your problem.
The cure? Shoot the shit out of it with a standard carbine buffer, with 5.56 powered ammo and don't worry about it not locking back for a while, because after a while it will start locking back, as things wear in.
This is said being under the assumption that other than not locking back it otherwise functions fine.
Mysteryman
12-02-16, 01:08
14.5" and mid-length gas is your problem.
The cure? Shoot the shit out of it with a standard carbine buffer, with 5.56 powered ammo and don't worry about it not locking back for a while, because after a while it will start locking back, as things wear in.
This is said being under the assumption that other than not locking back it otherwise functions fine.
I don't see that being the problem based on the brand of the upper. I'm confident BCM knows what they're doing. My 14.5" BCM middy has never not locked back or otherwise ran as it should. I know it's a sample size of one but I just don't see shooting it more being the solution to the problem.
MM
How did you come up with that answer?
14.5" and mid-length gas is your problem.
The cure? Shoot the shit out of it with a standard carbine buffer, with 5.56 powered ammo and don't worry about it not locking back for a while, because after a while it will start locking back, as things wear in.
This is said being under the assumption that other than not locking back it otherwise functions fine.
You need to troubleshoot this correctly. That means also having another lower available to test and see if it's the upper or something with the lower.
My gut tells me that there is something up with the lower.
Issue of importance- Is this a factory BCM upper?
You need to troubleshoot this correctly. That means also having another lower available to test and see if it's the upper or something with the lower.
This.
I don't understand how people are coming up with these bizarre solutions.
Be wise and use the process of elimination.
This thread may also be of interest to you: https://www.m4carbine.net/showthread.php?88910-PMAGs-not-locking-bolt-open-figured-out-why/page5
I tried different uppers on this lower and the catch won't hold open. A few eliminations: It's not the buffer tube. It's not the gas system. The problem is something is out of spec I believe. That's why I'm trying to find technical specifications for the bolt catch. In the mean time I ordered a new lower kit from Brownells minus the trigger group. When that comes in I'm going to put a mic on the parts and compare to the malfunctioning bolt catch. If that doesn't solve the problem I'm thinking the lower is the problem.
I'm having the same problem with my latest build. I pull the charging handle back, engage the bolt catch by hand and it holds open until the rifle gets "jarred". This happens if I give the rifle a good shake or sometimes when closing the charging handle. This is not an issue with any of the other AR's I built or the factory built HBAR. What are the correct measurements from the bolt catch to the lower receiver when at rest and when deployed? I'm thinking the catch may be out of spec. It was a DPMS lower kit from Brownells.
Is the bolt catch spring too strong or mixed up with the disconnector spring?
Recall, the spring holds the catch down.
It should take more than a good shake, something like a gentle "mortar", to release the bolt.
Wayne Dobbs
12-02-16, 14:06
I was wondering if you have disassembled your bolt catch assembly and verified that you have both the bolt catch spring and plunger installed. Spring goes in first and plunger nests in it. Despite all the collective knowledge here, it's an example of the fact that cyber trouble shooting is nothing like having the gun in hand.
How did you come up with that answer?
Personal exp.
I own a factory BCM 14.5" mid-length upper with a pin/weld flash hider. I bought it lightly used from a co-worker, and semi- cheap, because the previous owner was having short stroking problems with .223, and it was rarely if ever locking back on an empty mag with either 223 or 5.56 powered ammo running a H buffer. Claimed he had about 200 rounds through it, and looked just like new to me. I made sure everything was OK with the BCG, and it did the same on me, on two different lowers, both with H buffers and standard wolff carbine springs. I switched to a regular carbine buffer, and it started locking back with 5.56 some of the time, stopped short stroking as often with 223. SO...............I am not a retard, and proclaimed the rifle under gassed. But before I started yanking off FS bases, or opening up gas ports, I went out and shot 600 rds of xm193 one weekend, running it wetter than wet, getting things nice and hot, and dirty, doing a few mags dump etc... and low and behold, it loosened things up, it locks back 100% of the time with 5.56 ammo now, does not short stroke with 223, and locks back most of the time with that too. I still have the carbine buffer in it, and about 1500 more rounds through it, and that issue has never popped back up. I think I'll put the H buffer back in it soon, maybe the next time I shoot it and see what happens.
I did a lot of internet informal research on 14.5" mid length gas systems while still trying to figure this thing out, and seen this exact issue pop up...........I wouldn't say "all the time" but often enough to make me think, that the 14.5" mid length can be a touch fickle in some cases.............kinda like a dissapator with a rifle length gas system, but not.
titsonritz
12-02-16, 16:31
14.5" and mid-length gas is your problem.
The cure? Shoot the shit out of it with a standard carbine buffer, with 5.56 powered ammo and don't worry about it not locking back for a while, because after a while it will start locking back, as things wear in.
This is said being under the assumption that other than not locking back it otherwise functions fine.
My BCM 14.5" middy runs fine and locks back on PMC Bronze, Wolf Gold and Tula, it has an H buffer and standard carbine spring. I kind of doubt that is the problem.
Are you by chance using a BAD lever? I added one to a working lower and began experiencing failure to lock back, even when using M855. Turned out there was a minor interference between the lever and lower which prevented the catch from rising completely. Just something else to look at.
You are partially correct. 14.5" can be finicky in their function (when you do not gas it correctly) or the gas system is interrupted (rings, key, gas tube, etc..).
That said, someone should have contacted BCM and allowed them to look at it. ANYONE can make a mistake. I know multiple people with 14.5" mid carbines and they always worked out of the box.
Personal exp.
I own a factory BCM 14.5" mid-length upper with a pin/weld flash hider. I bought it lightly used from a co-worker, and semi- cheap, because the previous owner was having short stroking problems with .223, and it was rarely if ever locking back on an empty mag with either 223 or 5.56 powered ammo running a H buffer. Claimed he had about 200 rounds through it, and looked just like new to me. I made sure everything was OK with the BCG, and it did the same on me, on two different lowers, both with H buffers and standard wolff carbine springs. I switched to a regular carbine buffer, and it started locking back with 5.56 some of the time, stopped short stroking as often with 223. SO...............I am not a retard, and proclaimed the rifle under gassed. But before I started yanking off FS bases, or opening up gas ports, I went out and shot 600 rds of xm193 one weekend, running it wetter than wet, getting things nice and hot, and dirty, doing a few mags dump etc... and low and behold, it loosened things up, it locks back 100% of the time with 5.56 ammo now, does not short stroke with 223, and locks back most of the time with that too. I still have the carbine buffer in it, and about 1500 more rounds through it, and that issue has never popped back up. I think I'll put the H buffer back in it soon, maybe the next time I shoot it and see what happens.
I did a lot of internet informal research on 14.5" mid length gas systems while still trying to figure this thing out, and seen this exact issue pop up...........I wouldn't say "all the time" but often enough to make me think, that the 14.5" mid length can be a touch fickle in some cases.............kinda like a dissapator with a rifle length gas system, but not.
I dis assembled the lower and turns out either the spring was too long or the well for the spring in the receiver was too shallow. Clipped a few turns off of it and everything functions as it should. Took it out and put a few rounds down range. Everything works as intended. Thank you everyone for the help.
You are partially correct. 14.5" can be finicky in their function (when you do not gas it correctly) or the gas system is interrupted (rings, key, gas tube, etc..).
That said, someone should have contacted BCM and allowed them to look at it. ANYONE can make a mistake. I know multiple people with 14.5" mid carbines and they always worked out of the box.
Looking back on my statement of "14.5" and midlength is your problem"....it was a piss poor way of wording things on my part. I realize just because I had one that was not gassed properly, it doesn't mean they all are under gassed in nature.
I still stand by my statement that the OP should try a few things like swapping the H buffer for a carbine buffer, and go out and run it hard and wet for a bit before he gets too worried about it, as that cleared up the issue in my upper. If that doesn't help............by all means contact BCM and they will for sure help him out and make it right.
I am also still under the assumption that the OP's rifle is running.......other than the not locking back issue............fine, I didn't see where,or if, he said it was short stroking or having other issues.
Mysteryman
12-04-16, 00:15
Looking back on my statement of "14.5" and midlength is your problem"....it was a piss poor way of wording things on my part. I realize just because I had one that was not gassed properly, it doesn't mean they all are under gassed in nature.
I still stand by my statement that the OP should try a few things like swapping the H buffer for a carbine buffer, and go out and run it hard and wet for a bit before he gets too worried about it, as that cleared up the issue in my upper. If that doesn't help............by all means contact BCM and they will for sure help him out and make it right.
I am also still under the assumption that the OP's rifle is running.......other than the not locking back issue............fine, I didn't see where,or if, he said it was short stroking or having other issues.
Except the OP apparently found the problem and it wasn't with the upper. Like others here I too have had zero issues with my 14.5" middy from day one and I also run an H buffer. Just saying..
MM
Except the OP apparently found the problem and it wasn't with the upper.
Actually, I don't think the guy posting about trimming the spring was the OP. Even with that solution, it doesn't necessarily mean the issue was the lower instead of the upper. The guy with the similar issue piggybacking the thread could still have a rifle that is under-gassed. I'm assuming the spring he trimmed was the buffer spring, which reduces the amount of load required to lock the bcg back on the last round. Like with most gas issues, the problem is in the upper, and the solution is achieved by adjusting the lower.
titsonritz
12-05-16, 11:17
Actually, I don't think the guy posting about trimming the spring was the OP. Even with that solution, it doesn't necessarily mean the issue was the lower instead of the upper. The guy with the similar issue piggybacking the thread could still have a rifle that is under-gassed. I'm assuming the spring he trimmed was the buffer spring, which reduces the amount of load required to lock the bcg back on the last round. Like with most gas issues, the problem is in the upper, and the solution is achieved by adjusting the lower.
I got that it was the bolt catch spring he was referring to.
I got that it was the bolt catch spring he was referring to.
That's equally plausible. I've had buffers on the brain recently. Hopefully Kilbo can confirm what was done, as it might help the OP.
titsonritz
12-05-16, 12:59
That's equally plausible. I've had buffers on the brain recently. Hopefully Kilbo can confirm what was done, as it might help the OP.
I gathered it from this statement: "the well for the spring in the receiver was too shallow."
Mysteryman
12-07-16, 04:48
Actually, I don't think the guy posting about trimming the spring was the OP. Even with that solution, it doesn't necessarily mean the issue was the lower instead of the upper. The guy with the similar issue piggybacking the thread could still have a rifle that is under-gassed. I'm assuming the spring he trimmed was the buffer spring, which reduces the amount of load required to lock the bcg back on the last round. Like with most gas issues, the problem is in the upper, and the solution is achieved by adjusting the lower.
I missed the piggyback guy doing the answering.. Either way clipping coils off your recoil spring to make your rifle "run" is absolutely wrong. Your rifle should work with factory parts in unaltered condition. Same goes for the bolt catch spring or any spring.
MM
I missed the piggyback guy doing the answering.. Either way clipping coils off your recoil spring to make your rifle "run" is absolutely wrong. Your rifle should work with factory parts in unaltered condition. Same goes for the bolt catch spring or any spring.
MM
I agree clipping coils, on any spring, is not likely the correct solution. It might work, but that doesn't make it the best fix. Although, I do know of at least one mfg of recoil springs who says theirs can be clipped to adjust the load. Still, after hearing how others read the piggybacker's comment differently, I'm not certain which spring he clipped.
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