New BCG won't lock back after the last shot

[Bulletski]

I just finished up on a partial rebuild of one of my ARs.
I put in a new barrel, a new Aero BCG, an almost unused Aero upper and a new buffer tube kit that came with a spring that was about one inch longer than the old one. But I can't believe that has anything to do with the problem.
That's all the info that I think might have anything to do with the problem.

ANYWAY, the problem is that I went to the range yesterday for the first time to get get the gun sighted in. What happened was that the BCG wouldn't go back all the way to lock up on the last shot. The gun cycled just fine otherwise.
With no rounds in a mag and pulling back on the charging handle, the BCG locks up just fine.
I've examined the bolt catch and it "seems" to be OK.
The only thing I can think of is that the bolt catch doesn't go high enough on the bolt face rim. However it seems to be high enough. That is, it's almost as high on the bolt face rim as on my other AR which works fine.

BTW, I tried different mags but with the same result. The BCG either wouldn't go back and lock up OR the BCG would just go back into battery after the last shot.

As always, thanks for replies

 

[C-grunt]

Im wondering if the gun is just undergassed enough that it will feed but not go back far enough to lock on the bolt catch?

Check for gas leaks. Also maybe try the older action spring. Unless the new buffer kit is an A5 system, then keep what it came with.

You could also be running into a tolerance stacking issue with the new upper.

It seems you have several possible issues that you're gonna have to work through to find the problem.

 

[Charlie98]

Also maybe try the older action spring.

It seems you have several possible issues that you're gonna have to work through to find the problem.

That, and that.

Anytime you make major changes to an AR, and you start having problems, you have to start reversing the changes one at a time until you find the cause. You changed a bunch of stuff... including the 1" longer recoil spring.

 

[jmorris]

Anytime you make major changes to an AR, and you start having problems, you have to start reversing the changes one at a time until you find the cause. You changed a bunch of stuff... including the 1" longer recoil spring.

Or you could just make one change at a time, from the start. This would not only allow you to instantly know what change is at fault, it lets you independently quantify improvements or not.

If it mechanically functions when you pull it all the way back, it wouldn’t be the first time a firearm had just enough gas to run but not enough to catch.

 

[Laphroaig]

Does it lock back if you manually cycle it on an empty mag? If so, its probably not the bolt catch.

Are you running a BAD lever? I had a similar problem and finally figured out the extra inertia was too much for my system.

 

[Charlie98]

Or you could just make one change at a time, from the start.

But the OP is past that...

 

[Bulletski]

Does it lock back if you manually cycle it on an empty mag? If so, its probably not the bolt catch.

Are you running a BAD lever? I had a similar problem and finally figured out the extra inertia was too much for my system.

What is a "bad" lever?

 

[N555]

Bring all the old parts back to the range.
Try switching back to the original setup one part at a time.

 

[C-grunt]

What is a "bad" lever?

Battery assist device. Its a lever that attaches to the bolt catch latch and gives you the ability to work the catch from the trigger guard.

 

[Bulletski]

Thanks for the replies. I'll backtrack on the parts.

 

[BreechFace]

Probably undergassed...
1. Check gas port alignment
2. Check gas key for leakage
3. Gas block leakage
4. Gas port size not in spec
5. Gas rings not in spec, or carrier not in spec (unlikely)

Or needs break-in using m193, m855 spec ammo and good lubrication. This is what I break in my new builds with until everything settles in.

What ammo is giving you these issues?

You sure you have a carbine buffer spring in there and not a rifle spring?
Carbine normal length is around 10-1/16"
Rifle normal length is around 11-3/4"

 

[LiveLife]

new barrel, a new Aero BCG, an almost unused Aero upper and a new buffer tube kit that came with a spring that was about one inch longer than the old one

BCG wouldn't go back all the way to lock up on the last shot. The gun cycled just fine otherwise.

I had similar issue with a 20" rifle length gas tube build that would not cycle the bolt to eject spent cases. (rifle length has smallest gas port diameter than middle/carbine/pistol length but produces milder recoil)

Tried different length/rate recoil springs with no change. Checked the gas port diameter thinking it was under gassed and port was minimal diameter of 0.093" for rifle length - https://tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html

Cause turned out to be slightly canted gas block as after reassembly checking gas port diameter, AR cycled no problem and locked back after last round.

So, do a thorough cleaning, make sure the bolt gas rings are well lubed and lugs are dripping wet, and properly align the gas block to gas port to not cant (use alignment marking with marker during reinstall) and use one round in the magazine to check last round bolt hold open.

If you are shooting reloads loaded lighter, I found new tighter lug engagement could use some "break in" so I usually run factory/M193 to break in the bolt to barrel lug engagement surfaces.

 

[jmorris]

But the OP is past that...

Yeah but you know as well as I do, with AR’s, there is always a “next time”...

 

[TonyAngel]

Your new spring being 1" longer than the old spring is likely because the old spring has been used and has compressed. If you can manually lock the bolt back, meaning that the bolt can travel back far enough to lock back, then you're likely fine on that point.

What barrel are you using? What ammunition are you using? A barrel with a conservative gas port intended for 5.56 and under powered .223 ammo are not a good combination. This can and will cause exactly what you are describing.

I am, of course, assuming that you installed the gas block directly over the port. Some blocks are intended to be used with handguards that require a handguard ring and some are intended to be pushed right up against the shoulder on the barrel.

 

[small town life]

What I need to know is what cartridge is it! After that I can fix your issue

 

[small town life]

I need to know gas system length, barrel length, carbine or rifle buffer system also.

 

[Triggernosis]

Does it lock back if you manually cycle it on an empty mag? If so, its probably not the bolt catch.

Are you running a BAD lever? I had a similar problem and finally figured out the extra inertia was too much for my system.

I was going mention the exact same thing.