Frankenstein AR-15 troubleshooting question

[sparkstable]

New gun will cycle and hold the bolt open on the last round when resting the gun on the mag for stability. When shooting form that position everything is amazing.

But when standing, the bolt will not hold open on the last round. It cycles through ammo fine, just won't hold open on the last round when I'm standing.

I tried 4 mags and 2 types of ammo (Winchester m193 white box and PMC X-Tac 55gr).

The magazine follower clearly engages the bolt catch and can be seen when taking the gun apart (looking at just the lower). The mag does not slide up or down any, either, once seated (I pulled and pushed on it while the lower had been separated from the upper so I could see what the magazine was doing). There really was no movement up/down which makes me think it isn't an issue of the gun being rested on the mag that causes the catch to work versus standing (although, I am totally OK with being wrong... I'm still learning).

It has an 11.5" barrel, non-adjustable gas black, ToolCraft BCG and a Super 42 H3 buffer/spring. Both receivers are Aero. Bolt catch and mag release were from a CCMG LPK.

Any ideas what may be causing the issue?

Sorry if I posted this in the wrong place.

Thanks for any help/guidance anyone can offer. I'm still a bit of a novice (especially with the mechanical aspects of an AR) and always trying to learn (and constantly discovering just how little I know!).

 

[Gideon]

Will it hold open when you cycle the bolt manually on an empty mag, or is it only doing that when you're shooting?

 

[sparkstable]

You mean without me pushing on the bolt catch? Just rack the slide back with a mag in the well? I will try that tomorrow after work to see.

But it does hold open when firing from a rested position (resting on the mag), for what that is worth.

 

[dennishoddy]

It sounds like you're under gassed. When I set up an adjustable gas block, (that you don't have) its adjusted down until the BCG won't lock open on the last round, then opened up 1/2 round.
It appears to me that your right at that breaking point where the gas is just enough to lock open when resting the rifle on the mag but not when firing offhand. There is a tiny bit of movement in that mag when firing that is the issue as when resting on that mag, it's pushing further up vs offhand when it's hanging.
My suggestion is to go to a variable gas block so you can tune it.

 

[O4L]

Have you tried different brands of magazines?

 

[zipty6]

It sounds like you're under gassed. When I set up an adjustable gas block, (that you don't have) its adjusted down until the BCG won't lock open on the last round, then opened up 1/2 round.
It appears to me that your right at that breaking point where the gas is just enough to lock open when resting the rifle on the mag but not when firing offhand. There is a tiny bit of movement in that mag when firing that is the issue as when resting on that mag, it's pushing further up vs offhand when it's hanging.
My suggestion is to go to a variable gas block so you can tune it.

Great observation. It could also be as simple as a misaligned gas block.

 

[Gideon]

It sounds like you're under gassed. When I set up an adjustable gas block, (that you don't have) its adjusted down until the BCG won't lock open on the last round, then opened up 1/2 round.

Could it also be the H3 buffer if he has a reasonably sized gas port?

An adjustable gas block won't make his gas port larger if he's already undergassed.

 

[Seadog]

Could it also be the H3 buffer if he has a reasonably sized gas port?

An adjustable gas block won't make his gas port larger if he's already undergassed.

I was going to mention that also. H3 is heavy, doesn’t sound right. Should probably be a H1

 

[MilitantBEEMER]

Is the mag release button threaded far enough? If not , it may not grab the mag snug enough which could be why when resting on the mag it works fine? Just a novices thoughts at 6:30 in the morning with only a little coffee consumed

 

[TeamTomlyn]

Your movement under recoil standing up, like limp wristing a semi auto pistol, dampens the rearward travel. That at least explains the difference in stance; if you're not having mag problems.
I would first try a lighter buffer. If that doesn't solve it reset your gas block and make sure it and your gas port are not impeded in some way. If those cheaper options don't fix it get an adjustable gas block.
Actually do 1 & 2 anyways and get the adjustable gas block. If you are running a short barrel these 3 things are essential for reliability