Shooting my nephew's Colt A2 this weekend it has a 14" bbl and a carbine system on it, when you fire, the bolt will unlock, and kick back about 1/2', but thats it, no further. It won't eject and pick the next shell up. When he got the gun several weeks ago, it was working fine without issue, then it just stopped ejecting after about 40-50 rounds. I stripped the gun down and cleaned it very thoroughly, because it was pretty nasty, and figured that was the problem, but no luck. I have tried everything to try to get it to work, changed lowers, changed BCG, etc. I haven't checked to see if the gas ports are aligned, but I wouldn' think it shifted because it is an older upper, and looked pretty solid.

Does it cycle by hand ok?

If it does, and you changed the lower and the BCG, then I would look at the gas system.

Check that the gas tube is aligned in the upper properly and it fits with the key.
Check that the gas tube is in the proper position in the gas block and it is indeed pinned.

My off the wall guess is that the gas tube was not pinned in the gas block and has backed out.

+1

Definitely sounds like a gas issue.

Did you remove the buffer spring and oil/grease the interior of buffer tube? Some can be very dry and hang up the spring from recoiling properly. I use a small amount of synthetic wheel bearing grease on my buffer's tube interior as it won't burn off under any usage and stays where it's put.  

Other thing is running a cleaning swab dripping wet with some Hoppe's bore cleaner down the gas tube to make sure it's not carboned/plugged up somewhere at the barrel port hole. Let sit couple minutes, then use some computer canned-air and put the nozzle down the gas tube and blow out. You'll also want to place a rag around the gas tube's end as you will have some air blow-back and the rag will absorb/capture any grit you may find. Look down barrel tube to see if anything is now in barrel that needs cleaned out.

You already did a check/change out of components, so there isn't anything left other than shifted gas block---but I doubt if it's pinned on. It can only be a plugged hole with maybe some metal/lead shaving blocking port.

 

My first thought is that it is a gas issue as well, almost like it is getting alot of blow by.

I checked to make sure it fits with tube alligned with the gas key, when I changed out the BCG. I call myself looking at the gas block, to make sure it was pinned, and I am pretty sure it is, but will have him check again.

I cleaned the gun well, including blowing out the gas tube with brake cleaner to get any gunk out.

Like I said, the only thing I haven't checked is the alignment of the gas block on the barrel with the barrel ports.

I don't have the gun with me to inspect it any further, but it sure left me scratching my head.

***After I got to thinking about it, and just talked to him, he bought him a bump fire systems stock, and the issues started after he ran the first mag through it on auto with the bump stock. That makes me believe that he might have move his gas block, and everything is now misaligned, and not getting enough gas through to kick the bolt open.***

Its a gas issue no doubt.

Check gas rings, gas key, gas tube, and the gas block.  Those are really the only issues it could be.

Swapping the bolt carrier group is one simple/fast way to diagnose.  If it works, its either the key or the rings.  If it doesn't, its the block or the tube.....

I've been having a similar issue. My stag LH won't eject empties. I've checked my gas block, tried new ammo, different mags and cleaning/lubing everything and nothing has helped. I'm told it could be the bolt. Is there anyway to check my bolt for a problem? I don't have another left handed BCG to swap it out with. If its not that I can only assume its an issue with the gas port.

Any ideas?

Colt A2 would have come with an A2 fixed stock, and had an action spring about 13" free length, and a rifle-type buffer that's about 6" long. Bump-Fire stocks are carbine-length, so we can assume that the gun has a carbine-length receiver extension: is it possible that the rifle-length spring and buffer got stuffed in the carbine tube?

DO NOT DO THIS.  REPEAT.  DO NOT DO THIS.  IT COULD BE FATAL.

Brake cleaner, when heated, can decompose to Phosgene Gas.  This is really bad stuff.  It put a welder in the hospital, and he wrote about it.

That was while welding, but gas tubes can get pretty hot.  After reading that article, and doing some other reading on the topic, I banned brake cleaner from the shop and household.

Carb cleaner is ok, but generally I stick to Acetone or Isopropyl Alcohol.  Never really needed more than that.

I would check the gas tube.  It might have lost the pin and shifted out of place or rotated some.  Make sure it didn't split, allowing most of the gas to vent somewhere besides into the gas key.  Check the bolt rings.  They may simply be worn the hell out.  Check the gas key.  The bolts may be loose.  Make sure the passage in the gas key is clear.  Make sure the tube on the gas key itself isn't cracked.

And most importantly.  What kind of ammo are we talking?  Is it factory ammo or bubbas reloads with half a powder charge?  Were the bullets hitting the target when fired?

First thing I'd do would be to bring the rifle back to original factory configuration. Loose the bump-fire stock. Start over.

Also one poster said "an A2 comes with a fixed stock".

I own a carbine gassed Colt "AR15-A2 GOV'T CARBINE". Proving that it doesn't have to be a rifle with a fixed stock to be an "A2".

According to the article your statement as posted is inaccurate. Phosgene is only produced when Argon gas is also present.
In any case letting the brake clean or something such as carb cleaner evaporate before firing the rifle would alleviate safety concerns.

If you are concerned about cleaning chemicals Acetone is not a wise choice either.

Acetone is on the Right to Know Hazardous Substance List because it is cited by OSHA, ACGIH, DOT, NIOSH, IRIS,
NFPA and EPA and is also on the Special Health Hazard Substance List.  

Since you have tried a different lower and BCG that would seem to point to the tube, gas block, obstruction, or alignment.
You could try plugging and pressurizing to see if air comes out where it should not.