AR15 Bolt Doesn't Lock Back After Last Round

[FalArAk]

Decided to build up a lower I have had laying around for a few years.

"New" lower had the bolt catch already installed.

"Old" lower, no problems with the bolt locking back after the last round.

1) "New" lower using the same 20 round aluminum magazine and ammo as the old lower.
2) Bolt will lock back if I only fire one round.
3) Bolt will manually lock back with or without a magazine.
4) No changes at all to the upper receiver.
5) Using the same Geiselle H2 buffer and extra duty super enhanced Mkultra spring in the same buffer tube and stock.
6) With bolt locked back, it won't fly forward if banged on. IOW, like hitting the receiver with my palm, banging the stock against the ground won't release the bolt.

It is obviously the lower, what can I check or adjust?

 

[jayhitek]

You mentioned the bolt catch was already installed.. maybe the pin and spring were installed wrong?
Or, Is it possible, the magazine is not sitting high enough or in the correct spot..? And there is something wrong with the Mag release button and mag catch. if the bolt thinks the magazine is empty.. it will lock back. And if yours has a round in it and it's not chambering the next round.
Try pulling your magazine out without pressing the mag release button. Then try a totally different magazine.

 

[ThePreBanMan]

For shits and giggles, did you try more than 1 mag?

 

[FalArAk]

You mentioned the bolt catch was already installed.. maybe the pin and spring were installed wrong?
Or, Is it possible, the magazine is not sitting high enough or in the correct spot..? And there is something wrong with the Mag release button and mag catch. if the bolt thinks the magazine is empty.. it will lock back. And if yours has a round in it and it's not chambering the next round.
Try pulling your magazine out without pressing the mag release button. Then try a totally different magazine.

Using a different mag won't help me figure out the problem, the magazine works fine, just not with this upper for some reason.

I think if the mag wasn't in the right spot or bolt catch was not installed(by the factory) correctly, it wouldn't lock into the receiver or feed correctly etc

Mag release works as it should, feeds ammo fine, locks into lower fine.

I shot 60 rounds and replicated the problem every time.

 

[FalArAk]

For shits and giggles, did you try more than 1 mag?

Too easy.

No, I would have if I had another mag at the time though

I want to try and figure out the problem, with everybody's help lol

 

[FalArAk]

I am using a different buffer tube, but they are both "milspec" and 6 positions.

 

[jayhitek]

This is a lower you built? Is this a 80%? Are you sure the holes are to factory spec or is this a factory made lower?

 

[FalArAk]

This is a lower you built? Is this a 80%? Are you sure the holes are to factory spec or is this a factory made lower?

No, it's a factory lower.

For the sake of this discussion, lets assume the problem persists with other mags as well.. Which I honestly believe is the case. I have been using this 20 round lower for years with many different AR's.

 

[jayhitek]

OK so what is your process of how to get it to fire again?
You load the magazine, charge the handle, fire, bolt locks back.. Now what? Drop the mag..? and how do you get the bolt into battery to open the upper?

And is this a standard 223 lower and not a 9 or 45 or something else?

 

[Spartan65]

It’s almost certainly a mag issue

 

[Spartan65]

Give a tap rack next time you’re at the end of the mag. She will probably lock

 

[FalArAk]

OK so what is your process of how to get it to fire again?
You load the magazine, charge the handle, fire, bolt locks back.. Now what? Drop the mag..? and how do you get the bolt into battery to open the upper?

And is this a standard 223 lower and not a 9 or 45 or something else?

Release mag, lock bolt back, insert mag, release bolt and shoot.

Normally, step two isn't necessary.

Yes, standard Ambi LMT Multi-Cal.

 

[FalArAk]

It’s almost certainly a mag issue

It worked perfectly for the last 10 years on various lowers.

It has to be something with the bolt catch, I just don't know enough about them to figure it out and I have NEVER had a problem like this before. The lower that this replaced I put it together as well.

 

[FalArAk]

Give a tap rack next time you’re at the end of the mag. She will probably lock

I can pull the charging handle right now and it locks back without me depressing the catch.

I can't remember if it is supposed to do that or not lol

As a habit, I always used to push the bolt catch in while pulling the charging handle back to lock the BCG into place.

 

[whatluck]

I can pull the charging handle right now and it locks back without me depressing the catch.

I can't remember if it is supposed to do that or not lol

As a habit, I always used to push the bolt catch in while pulling the charging handle back to lock the BCG into place.

That's not right

 

[FalArAk]

That's not right

So bolt catch might have a spring that is too strong or something?

 

[greencobra]

just replace the old, installed mag catch assembly, you know, just for yucks. it's not like it's high dollar parts we're talking about. i'm also thinking the person who installed it didn't wind it in as far as it would go before that piece hit and scratched the receiver so it left the spring weakened.

 

[FalArAk]

 

[FalArAk]

Here it is with hammer up, it is hitting the bolt catch..Guessing this isn't normal?

 

[FalArAk]

I can press down on bolt catch and it goes down a hair making the hammer barely clear it.. Is this how it should look ? When I press the bolt catch down, it also slightly depresses the follower.

 

[FalArAk]

just replace the old, installed mag catch assembly, you know, just for yucks. it's not like it's high dollar parts we're talking about. i'm also thinking the person who installed it didn't wind it in as far as it would go before that piece hit and scratched the receiver so it left the spring weakened.

It's an ambi bolt catch release, don't have one hanging around

 

[not new guy]

The bcg should be stopping the hammer from hitting when it’s assembled though, no?

 

[FalArAk]

Ehh, forget that.. Just inserted the mag into the old lower and it did the same thing and looked the same way.

 

[FalArAk]

The bcg should be stopping the hammer from hitting when it’s assembled though, no?

Ya, probably.. I am just desperately searching for a solution now lol

 

[FalArAk]

I think maybe I figured it out? On the right side of the receiver the ambi catch can be pushed outward from the top kinda keeping the mag catch in the high position. Press it in and the catch goes into the lower position. This is a pic when it is keeping the catch in the upper position.

 

[LakeTrout]

I may not understand your post correctly. It if your bolt won’t lock back on last round and you are using an H2 buffer and extra power spring, you’ve got too much resistance back there. Why are you adding all of that weight and spring tension? You’re out of spec. The first thing I would do is put in a standard spring and buffer.

 

[FalArAk]

I may not understand your post correctly. It if your bolt won’t lock back on last round and you are using an H2 buffer and extra power spring, you’ve got too much resistance back there. Why are you adding all of that weight and spring tension? You’re out of spec. The first thing I would do is put in a standard spring and buffer.

H2 buffer for 16", no?

 

[FalArAk]

I think that was it, I can no longer pull the charging handle back and it auto locks in place, I have to manually do it like before.

Guess I am going to the range tomorrow lol

Apparently ambi controls are too advanced for my neanderthal mind

Damn, that isn't it.. I put the mag in and it locks back automatically... I am done for the day.

 

[LakeTrout]

H2 buffer for 16", no?

What length gas system?

This can go down a rabbit hole but assuming your gas port is mil spec and you’re using a carbine buffer tube, you’d use a carbine buffer.

That said, Lots of people step it up one because manufacturers nowadays drill gas ports out larger than in the past to get the gun to run on every shitty brand of ammo to boost their reliability ratings. So most off the shelf ARs are way over gassed. A “fix” for this, although not the best one, is to run a heavier buffer to soften or slow down the recoil impulse. H2 is still pretty heavy and a couple steps up. Combine that with an extra power spring and you’re asking for issues unless you have a shorter gas system with an over-bored gas port and you’re way overgassed.

Lots of people get away with doing stuff like this in the short run because they don’t shoot much. But if they shot enough to get the thing dirty, or shot in various weather conditions, they often start to see issues.

 

[FalArAk]

What length gas system?

This can go down a rabbit hole but assuming your gas port is mil spec and you’re using a carbine buffer tube, you’d use a carbine buffer.

That said, Lots of people step it up one because manufacturers nowadays drill gas ports out larger than in the past to get the gun to run on every shitty brand of ammo to boost their reliability ratings. So most off the shelf ARs are way over gassed. A “fix” for this, although not the best one, is to run a heavier buffer to soften or slow down the recoil impulse. H2 is still pretty heavy and a couple steps up. Combine that with an extra power spring and you’re asking for issues unless you have a shorter gas system with an over-bored gas port and you’re way overgassed.

Lots of people get away with doing stuff like this in the short run because they don’t shoot much. But if they shot enough to get the thing dirty, or shot in various weather conditions, they often start to see issues.

It ran perfectly before, it is the exact same upper.

The new buffer tube is LMT(I like the indexing for the detent pin) and the old one was Geiselle.