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Feeding issues. Doesn't wanna eat.

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DanKATL DanKATL is correct. You can manipulate the bolt manually with no issues because your problem isn't mechanical...it's the gas system. Something is "out of balance". Either it's undergassed or the buffer is too heavy. The bolt isn't going all the way back, therefore not catching the next round (or not properly) and the last round bolt hold open isn't happening because it's also not going back far enough to catch the latch. I would say if you want to swap uppers around like you're doing, get an Odin adjustable buffer and then you could tune it for whatever upper you want at that particular time.
Looking online now at adjustable buffers. Seems to be the easiest solution. Unless its not. Either way it seems to be a good place to start
 
My guess without seeing it is exactly what folks have said above; she's undergassed.

You could have an undersized gas port in the barrel.

Gas block could be misaligned, or out of spec.

Gas tube could have a blockage, or be out of spec.

Bad seal in the carrier at the bolt, due to an oversized bore or bad gas rings.

Losing gas at the gas key due to being improperly installed, or mating surfaces aren't even or sealed.

The bore in the gas key could be oversized and not sealing around the gas tube.

If you bring it by TruPrep in Marietta, I can diagnose it.


Worth a shot; have you tried different loads through it? Could be weak/out of spec ammo.
 
Your lower is fine. Any lower that will run on a carbine upper will run on a Midlength upper.

Your buffer, no matter if it’s a standard or “H” is fine. H buffers are made to prevent bolt bounce in a carbine firing full auto. Switching to a lighter buffer treats a symptom not the problem. If you don’t have a rifle length buffer spring in the receiver extension (I know, sounds crazy, but I’ve seen it)….

You have a gas problem. That upper is under gassed. It could be an out of spec:

a). gas tube (worn or misaligned)
b) bolt (bolt tail, gas rings)
c) bolt carrier (inner diameter of bolt tail opening worn or too large, gas port)
d) carrier key (worn or misaligned or bolts loose)
e) barrel gas port

The most common gas issue I have seen on a new properly QCed and quality parts carbine has been misaligned gas tubes due to improper barrel nut alignment. A slightly misaligned barrel nut will cause the gas tube to be angled when it makes the transition from the nut into the receiver. That can put a varying amount of drag on the tube/carrier key interface, resulting in a functional gun with accelerated tube wear all the way to a gun that short strokes.

The second most common issue is worn gas rings or gas tube that simply needs to be replaced.

Third? Any of a combination of everything listed. Worn carrier keys and gas tubes are a high round count issue only quality carbines. By the time a select fire M4 that has seen a good amount of semi and full auto fire has a gas tube/carrier key worn to the point it causes malfunctions the barrel is close to needing to be replaced too.

When it comes to not properly QCed cheap parts/rifle uppers/lowers the problems can be so small, abundant, stacked up that finding the root then correcting the issue sometimes just isn’t worth the squeeze.

First I would make sure the bolt carrier key bolts are properly torqued and staked. Then I would strip the bolt carrier group down to bolt carrier assembly only. Then with the charging handle removed from the upper, slide the carrier into the upper and make sure it closes on the gas tube with no resistance. Then check that the gas tube flange isn’t excessively worn ( you will have to remove the tube to get a good look). Then I would replace the gas rings (it’s such a cheap fix I wouldn’t even waste my time diagnosing if the rings are worn). If that didn’t fix it I would take it to a certified AR armorer and let him figure it out and fix it.
 
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