View Full Version : Need help with my AR, Bolt not locking back after last round
Hello fella AR enthusiasts. I have a minor issue that I'd like your help/input on.
My AR's bolt does not lock back open after the last round. I use all P-mags and mostly American Eagle .223 ammo 55gr. The AR is a Spikes Tactical lower with a Rguns upper. Last range session I used both a brand new and a used Pmag and the bolt did NOT lock back on both mags.
I'm not sure if this is a ammo issue, or the bolt lock lever not engaging, or something else (under-gassed?). I doubt it is the magazines though, as I've used Pmags in my other AR's with no problem.
Mods please move to the appropriate section if needed. thanks for any help!
hogfan1911
10-29-12, 18:42
Just because the mags have worked in other rifles doesn't mean they'll work in this rifle. I'd still try some other brand of mag first before trying anything else.
Redbeardsong
10-29-12, 18:51
Are you running an H buffer? Full Auto Bolt Carrier? Perhaps you need a lighter buffer.
What buffer are you using? Are you running a BAD lever or equivalent?
Just because the mags have worked in other rifles doesn't mean they'll work in this rifle. I'd still try some other brand of mag first before trying anything else.
Why would I need to use different mags? Like I said, I used a brand new Pmag along with an older used Pmag with the same effect in this M4.
Also, sometimes it will lock back on certain occasions. I lube my AR very well and function check it, including the mags. The bolt hold-open lever engages on an empty mag upon inspection when the lower is disassembled from the upper.
I'm using a standard carbine buffer, no BAD lever.
The reason you try different mags to to see if there is a possibility that your lower/upper combo doesn't work with Pmags, which they apparently don't..
Does the bolt carrier lock back when you have just an empty mag and cycle the bolt carrier by hand?
Just for fun, next time you go shoot, borrow a few GI mags from a friend and see if the problem goes away..
Was the RGuns upper built by them or did you put it together yourself? Could it be that the gas block port is misaligned with the port in the barrel?
Blazingace
10-29-12, 20:41
There is a trick that you can do to check if it is the mag.
It is either the mags, the bolt catch or your gun is "short stroking".
There are probably dozens of threads on short stroking search them.
Hold the bolt catch to the "locked back" position I.E. press the lower portion of the bolt catch. Fire a round. If the bolt doesn't lock back you can see if the gun is short stroking. If it locks back then the bolt carrier is either short stroking I.E. not cycling far enough back. It might come back enough to strip the next round, but not enough to catch the bolt catch. Or over gassed, less likely, that the bolt carrier is moving too fast and the mag isn't fast enough to push the catch up. This would also cause you to have fail to feed (FTF) issues.
Gas port size, dirty gas tube, mis aligned gas block, weak ammo, loose gas key, damaged bolt rings. All these will cause short stroking.
Do the test above first. Get a friend to help if possible. See if it is the mags.
My brother had to send back 6 of his 20 Pmags for this issue. They ahot fine in my gun, but wouldn't work in his. We even swapped internals to a different mag body to experiment. It followed the internals.
The reason you try different mags to to see if there is a possibility that your lower/upper combo doesn't work with Pmags, which they apparently don't..
Does the bolt carrier lock back when you have just an empty mag and cycle the bolt carrier by hand?
Just for fun, next time you go shoot, borrow a few GI mags from a friend and see if the problem goes away..
Pmags have worked on this AR before with the bolt locking back on an empty round. I will also try to cycle it manually with an empty Pmag and see if it will lock ( i'm pretty sure it will).
There is a trick that you can do to check if it is the mag.
It is either the mags, the bolt catch or your gun is "short stroking".
There are probably dozens of threads on short stroking search them.
Hold the bolt catch to the "locked back" position I.E. press the lower portion of the bolt catch. Fire a round. If the bolt doesn't lock back you can see if the gun is short stroking. If it locks back then the bolt carrier is either short stroking I.E. not cycling far enough back. It might come back enough to strip the next round, but not enough to catch the bolt catch. Or over gassed, less likely, that the bolt carrier is moving too fast and the mag isn't fast enough to push the catch up. This would also cause you to have fail to feed (FTF) issues.
Gas port size, dirty gas tube, mis aligned gas block, weak ammo, loose gas key, damaged bolt rings. All these will cause short stroking.
Do the test above first. Get a friend to help if possible. See if it is the mags.
My brother had to send back 6 of his 20 Pmags for this issue. They ahot fine in my gun, but wouldn't work in his. We even swapped internals to a different mag body to experiment. It followed the internals.
Thank you for all the suggestions. I will try the test you suggested.
I guess it could be a number of things eh....
Also, can this problem be caused by worn out gas rings? My bolt somehow likes to chew up gas rings occasionally. I'm currently looking to buy a better BCG to see if this problem persists.
Was the RGuns upper built by them or did you put it together yourself? Could it be that the gas block port is misaligned with the port in the barrel?
It is a complete Rguns upper, mated to a Spikes lower that I built.
I am going out on a limb here. I will be willing to bet that it is overgassed. Rguns generally sells trashy AR stuff of questionable origin. Unless they have changed something.
I would probably try an H buffer and I think it will solve the issue. Just call me crazy.....
If your bolt is indeed tearing up gas rings, I'd say you answered your own question.
I don't know that I would suggest borrowing someone elses bolt to try w/o head space gauging it first. Who knows what condition your barrel extension lugs are in.
constructor
10-30-12, 19:53
Hello fella AR enthusiasts. I have a minor issue that I'd like your help/input on.
My AR's bolt does not lock back open after the last round. I use all P-mags and mostly American Eagle .223 ammo 55gr. The AR is a Spikes Tactical lower with a Rguns upper. Last range session I used both a brand new and a used Pmag and the bolt did NOT lock back on both mags.
I'm not sure if this is a ammo issue, or the bolt lock lever not engaging, or something else (under-gassed?). I doubt it is the magazines though, as I've used Pmags in my other AR's with no problem.
Mods please move to the appropriate section if needed. thanks for any help!
Try different ammo, FE ammo is low powered. Fed XM193 and WWB is pretty close to full power loads.
FeltaDorce
10-31-12, 14:45
Are you using 20 rd Pmags? I have had no issues at all with 30 rd Pmags for last round lock back, but my 20 rd Pmags will fail to lock back about 80% of the time. It is due to the tilting follower in the square 20 rd mags, and is "normal" albeit completely unacceptable.
My father's double star m4 upper had the same problem. I removed the front sight and measured the gas port. Turns out it was smaller than the spec dimension. After a minuet under the drill press, the gas port dia was up to spec. Now the rifle runs fine. I don't remember off the top of my head what the correct size is. I'm sure you can find it on AR15.com.
That is not a magazine issue!
Try different ammo, FE ammo is low powered. Fed XM193 and WWB is pretty close to full power loads.
Absurd...
An AR that won't run AE is problematic. Wolf... maybe. But AE223 is right down the middle lane.
some of the suggestions here are bordering on ARFcom levels of chasing your tail..
Over gassed is more likely when you're talking aftermarket ARs. A stiff bolt catch spring and a bolt group moving too fast will cause this. Definitely try a heavier buffer. The H2 most closely matches a rifle buffer's physics... and that's what you're after.
What direction is your brass ejecting? I know the ejection pattern is not gospel but it could help determine if it is overgassed if the ejection is forward, undergassed if ejecting rearward.
What direction is your brass ejecting? I know the ejection pattern is not gospel but it could help determine if it is overgassed if the ejection is forward, undergassed if ejecting rearward.
Direction can be a clue... but also how vigorously is the brass ejecting?
Why would I need to use different mags? Like I said, I used a brand new Pmag along with an older used Pmag with the same effect in this M4.
Sometimes, it really is the magazine. Read this (https://www.m4carbine.net/showthread.php?t=101687) and inspect. This is an easy problem to check for, and one I initially overlooked because of my blind faith in a known good company. If this isn't your problem, great, move down the list.
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