AR15 BCG won't lock back

[selfshrevident]

Some background: I just finished installing a new BCM 20" chrome lined barrel, BCM low profile gas block, and a Seekins nitrided gas tube. This was my first time doing any kind of gunsmithing.

Took her to the range today to test everything out and everything was functioning fine until I realized that the BCG was not locking back at all with any of the mags I was using. Prior to the new barrel assembly everything was working fine.

Some observations of mine: The brass was ejecting about 4-5ft (pretty healthy?). Every single case has a dent on the same exact spot now (this did not happen with old barrel assembly... too much gas/violent ejection?). Action cycled just fine, even when pulling the trigger as fast as I could for a 10rd dump.

I've read some posts on arfcom suggesting it could be a whole range of problems, with the alignment of the gas block the prime suspect. The BAD lever was also mentioned as a possible culprit but mine wasn't giving me any problems with the previous setup.

When putting the new barrel assembly together, I just backed the gas block all the way back against the shoulder, and that seemed to line up with the dimple on the bottom of the barrel. I used blue loc tite to secure the set screws. I did notice that the gas tube had/still has some play in the receiver- I can wiggle it with my finger... I was thinking this play would disappear over time after the heat & carbon cement it in place.

Any thoughts?

Edit: Forgot to mention I was using the same batch of ammo (reloads, 55gr FMJ w/ 25.3gr of AA2230) as I was using with the old setup. They ran fine before.

Update: Just finished cleaning it and noticed when dropping the BCG & charging handle back into the upper, the gas key is hitting/rubbing the gas tube. It's very subtle, but I can definitely hear it. I hope this straightens itself out over time... could this rubbing/friction be causing the BCG to move back so much slower that it doesn't lock back on the last round?

[prkprisoner]

Does your bolt release move freely? I bet it's too tight. Sometimes the finish on the receiver or bolt catch is a little too thick. Try to cycle the bolt catch about 100 times and see if it loosens up.

[selfshrevident]

Does your bolt release move freely? I bet it's too tight. Sometimes the finish on the receiver or bolt catch is a little too thick. Try to cycle the bolt catch about 100 times and see if it loosens up.

Seems to be moving freely with the slightest hint of grittiness. Maybe just needs a good cleaning. This wouldn't explain the dented brass though, or why it was functioning 100% before the new assembly

[kurby09]

Did your gas tube length changed? Dont quote me 100 % but from what i understand. Assuming you went from a mid or carbine length gas system to a rifle length being that you are using a 20" inch barrel. You may need to address your buffer and buffer spring as they are designed around gas tube lengths and power factors of the round. So many factors are to be considered i hope you are able to fix it.

[ar15barrels]

I did notice that the gas tube had/still has some play in the receiver- I can wiggle it with my finger... I was thinking this play would disappear over time after the heat & carbon cement it in place.

Any thoughts?

Edit: Forgot to mention I was using the same batch of ammo (reloads, 55gr FMJ w/ 25.3gr of AA2230) as I was using with the old setup. They ran fine before.

The movement of the gas tube within the barrel nut and the hole in the receiver is a GOOD thing.
That lets the gas tube self-center in the gas key.
This is what you are feeling when the carrier slides into the receiver.

Your load is pretty wimpy.
Perhaps your old barrel had a larger gas port.
I would warm up the load to around 26gr and I bet it starts to function better.

[pterrell]

As far as the bolt locking back, does the bolt catch engage when you insert an empty magazine? Magazine is almost always the cause for the bolt not locking back. Also, make sure that you have a rifle length gas system and a rifle buffer and buffer tube instead of a carbine one.

[kcstott]

4-5' ejection is whimpy every AR I've shot put the brass 10' away at least and slightly forward

[selfshrevident]

Did your gas tube length changed? Dont quote me 100 % but from what i understand. Assuming you went from a mid or carbine length gas system to a rifle length being that you are using a 20" inch barrel. You may need to address your buffer and buffer spring as they are designed around gas tube lengths and power factors of the round. So many factors are to be considered i hope you are able to fix it.

No the gas tubes are the same- rifle length. Previous barrel was a 20". I did switch to a magpul MOE carbine stock and tapco milspec carbine buffer tube assembly while I had the old barrel on, and everything functioned fine. Hopefully that's not the problem...

I'm going to fully clean and lubricate the rifle and magazines, then take it shooting again this weekend with some warm loads and report back.

[selfshrevident]

The movement of the gas tube within the barrel nut and the hole in the receiver is a GOOD thing.
That lets the gas tube self-center in the gas key.
This is what you are feeling when the carrier slides into the receiver.

Your load is pretty wimpy.
Perhaps your old barrel had a larger gas port.
I would warm up the load to around 26gr and I bet it starts to function better.

Glad to see that the movement is normal. I had a theory that after a lot of heat and carbon the gas tube would center itself in place.

I will try some warmer loads this weekend and report back.

[selfshrevident]

As far as the bolt locking back, does the bolt catch engage when you insert an empty magazine? Magazine is almost always the cause for the bolt not locking back. Also, make sure that you have a rifle length gas system and a rifle buffer and buffer tube instead of a carbine one.

The bolt catch engages on an empty mag. I have a rifle length gas tube but a carbine length buffer tube assembly... I'm hoping after a good thorough cleaning, lubing and warmer loads that it'll solve itself and I won't have to replace the buffer assembly.

[pterrell]

The bolt catch engages on an empty mag. I have a rifle length gas tube but a carbine length buffer tube assembly... I'm hoping after a good thorough cleaning, lubing and warmer loads that it'll solve itself and I won't have to replace the buffer assembly.

A rifle length gas system needs a rifle length buffer and tube. That is likely the cause of your problems.

[isplice]

A rifle length gas system needs a rifle length buffer and tube. That is likely the cause of your problems.

That is correct. What do those damn Canadians know anyway!

[valley82]

A rifle length gas system needs a rifle length buffer and tube. That is likely the cause of your problems.

Ummm, wrong... The two are not related, the barrel (profile) you choose determines gas system length. The stock type (buffer tube used) you choose determines buffer type/length.
Buffer weight could be an issue but that is a different issue.

[kcstott]

Ummm, wrong... The two are not related, the barrel (profile) you choose determines gas system length. The stock type (buffer tube used) you choose determines buffer type/length.
Buffer weight could be an issue but that is a different issue.

Yep and it's the spring, tube, buffer, combo that needs to match, gas system independent

Carbine buffer with carbine spring and tube,

You can run a carbine buffer in a rifle tube but how well it will function is another story,

You can not run a rifle buffer in a carbine tube the buffer will stop short.

[selfshrevident]

A rifle length gas system needs a rifle length buffer and tube. That is likely the cause of your problems.

I've read around and gotten mixed responses. Like I said it was fine before the barrel/gas system swap. I hope I don't have to take the tube off. I already staked it on