AR Bolt not Locking Back

B

bluetopper

New member
What's causing my bolt to sometimes not lock back after the last round in the mag in my M4 Carbine? When the bolt does lock back just the least little jarring of the rifle will send the bolt in to battery. It does this with all mags.

What can I do?
 
Ammo
Mags
Bolt catch
Buffer weight
Buffer spring
Could be any of the above as well as other things I didn't mention. What brand of rifle is it? Is it a put-together gun or a factory gun?
 
Underpowered ammo.

Buffer to heavy.

Those are the two things I would check first.


You aren't using one of those stupid "BAD lever" things that attaches to the bolt release are you? I have seen those things cause more problems than they fix.
 
Was this home built AR?
It could be that the buffer tube is screwed in too much.
The bolt would not travel far back enough to get a good catch on the bolt catch. Seen this happen before.
 
I've had this happen for two different reasons.

The first was on a Tactical Inovations billet lower and the cut out for the mag catch was not machined quite deep enough. When the catch would rock on the roll pin it would "bottom out" limiting the other end of the catch that actually holds back the bolt from sticking up completely. I ended up filing a little material off the back of the catch; it's a pain to do as you will need a roll pin punch to remove the catch.

The second case was where I was using an ACE skeleton stock which requires an A1 length bolt to attach it to the buffer tube. The A1 length bolt is only about 3/4" long. I was mistakeningly using a bolt meant for the A2 stock and it was longer and sticking into the buffer tube just enough to prevent the buffer from going back far enough to allow the bolt catch to grab the bolt.

In your case if you have the same problem with different magazines it's most likely a scenario where the catch isn't sticking up enough; you may need to file the back of the part like I did.

I think I have an extra one I filed at home; I can post it here to explain.

On a side note it is very common for the catch to let the bolt fly, if you bang the stock of an AR on the ground and there is no magazine in the gun.
 
You stated all mags. Are they good mags?
If yes, then is it doing it for all ammo type?
If yes then check bolt catch and the bcg, buffer and buffer spring.

All this is assuming you have cleaned the rifle.:)
 
Does the bolt lock back properly when you hand cycle it? If so, it's not the buffer tube,spring, or any of that stuff in the rear.
There may be a burr on the bolt latch lever or plunger or the slot may just be tight. The bolt latch is engaged by the mag follower against the pressure of the spring. If any of the connected parts is hanging up, the follower will not raise the bolt latch sufficiently to securely lock the bolt back.
If the bolt is not traveling far enough to the rear, the bolt latch may catch the bottom of the bolt carrier rather than the front of the bolt which will allow the bolt to slip off and fly forward if jiggled or bumped. In that case, you have a problem related to weak ammo, a plugged gas tube, or any of the problems related to the buffer assembly.
 
Try a different magazine, it could just be your mag. I know with my AR 7.62x39 i bought some "monster mags" which are AR15 tops welded to a AK47 bottom and it doesn't have the mag catch on there so after firing the last round in the mag, the bolt doesn't lock open
 
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