AR problem

[G19G26]

I've built another AR. This is the first one to give me trouble. All quality parts used. 18" barrel and Aero Precision M4E1 receivers.
IT doesn't want to fully cycle with Magpul mags. Seems to
cycle ok with metal mags. I can get it to cycle better with hotter loads, but want to develop most accurate loads.
I had this checked by gunsmith. He confirmed that everything looks right. He opened gas port by 0.010" and lightened buffer spring mass. I've put in the lightest buffer I have. ( 2.8oz).
Anyone have a solution we haven't thought of?
BTW, does this with factory ammo, too

 

[Elcardo]

Just curiosity but have you tried different pmags or do you only have 1 that you are trying ?
I'm sure you've probably tried multiple of them but had to ask just to clear up the possibility of 1 bad mag

 

[Tyler-The-Piker]

When you say "fully cycle" be more specific...where's the hang up? what's the end result? Is this while actually firing the rifle or just trying to function-check "cycle" rounds through it in your living room?

 

[OneBadV8]

how many rounds are in the pmags? Mine work the best with 28 instead of 30

 

[Gingerbeardman]

Magazine catch keeping certain mags too high so the bolt can't travel forward properly?

 

[DadSmith]

how many rounds are in the pmags? Mine work the best with 28 instead of 30

You run them GI style.

 

[DadSmith]

I've built another AR. This is the first one to give me trouble. All quality parts used. 18" barrel and Aero Precision M4E1 receivers.
IT doesn't want to fully cycle with Magpul mags. Seems to
cycle ok with metal mags. I can get it to cycle better with hotter loads, but want to develop most accurate loads.
I had this checked by gunsmith. He confirmed that everything looks right. He opened gas port by 0.010" and lightened buffer spring mass. I've put in the lightest buffer I have. ( 2.8oz).
Anyone have a solution we haven't thought of?
BTW, does this with factory ammo, too

Have you checked the buffer tube for burrs inside? Also did you oil the buffer spring? I found that to solve several issues in the past. Doesn't have to be dripping oil just a light coating.

 

[G19G26]

Just curiosity but have you tried different pmags or do you only have 1 that you are trying ?
I'm sure you've probably tried multiple of them but had to ask just to clear up the possibility of 1 bad mag

Yep, tried 3 different gen 3 20rnd, and a gen2 30 rnd

 

[G19G26]

When you say "fully cycle" be more specific...where's the hang up? what's the end result? Is this while actually firing the rifle or just trying to function-check "cycle" rounds through it in your living room?

It won't extract the case because it only partially retracts the bolt or the bolt doesn't lock back on empty chamber

 

[churchmouse]

how many rounds are in the pmags? Mine work the best with 28 instead of 30

Same here.

 

[churchmouse]

It won't extract the case because it only partially retracts the bolt or the bolt doesn't lock back on empty chamber

Have you swapped out the BCG with one from another rifle.
This may sound a bit off but I had the same issue last summer and pulled the BCG from another rifle (straight across swap) and both ran after that...

Same BCG's. Go figure.

 

[bwframe]

Have you swapped out the BCG with one from another rifle.
This may sound a bit off but I had the same issue last summer and pulled the BCG from another rifle (straight across swap) and both ran after that...

Same BCG's. Go figure.

I've seen this work more than once.

 

[churchmouse]

I've seen this work more than once.

I was doubtful myself. Same brand Nicol Boron BCG's.
Both rifles were spot on after the swap.
I pulled and measured both when I got home.
Only thing I come up with is the relation of the gas tube to the bolt carrier.

 

[G19G26]

I've seen this work more than once.e

I've seen this work more than once.

Even if loaded ionly loaded with 5 rounds

 

[bwframe]

I was doubtful myself. Same brand Nicol Boron BCG's.
Both rifles were spot on after the swap.
I pulled and measured both when I got home.
Only thing I come up with is the relation of the gas tube to the bolt carrier.

I've been lucky that its never been my rifle with these issues. I have supplied a test BCG that worked in another's problem gun.

I often wondered if doing a longer term one for one swap like that would eventually settle in and allow swapping back?

 

[churchmouse]

I've been lucky that its never been my rifle with these issues. I have supplied a test BCG that worked in another's problem gun.

I often wondered if doing a longer term one for one swap like that would eventually settle in and allow swapping back?

I have wondered the same. Never really attempted the experiment

 

[tcecil88]

To me it sounds like it is undergassed. Did opening up the gas port help any?
I put a cheap gas block on an M4 rifle when I converted it to a free float tube. It went from shooting everything to only cycling 5.56 reliably. I put a better quality gas block on it and that fixed it.

 

[G19G26]

To me it sounds like it is undergassed. Did opening up the gas port help any?
I put a cheap gas block on an M4 rifle when I converted it to a free float tube. It went from shooting everything to only cycling 5.56 reliably. I put a better quality gas block on it and that fixed it.

I'm using a generic adjustable gas block. It is full open.
Yes opening up gas port did help some.

 

[G19G26]

I have swapped BCG already. The only things I haven't swapped due to not having spares are the gas block and tube, and bolt hold open lever. When I get to range over the weekend, going to swap entire upper to a.known good lower

 

[G19G26]

I changed to the lightest buffer I had ( 2.8 oz) and a buffer spring that is 1 inch shorter and easier to compress. Took to range today. All is good. No fail to eject and every magazine locked the bolt back when empty.
Thanks to everyone who offered help. Good people here.