A buddy of mine just bought a parts gun AR: Bushy lower & who knows what upper. The lower is an A2 config but the upper is the short barrel M4 config, 16" (I think).

His problem is it only fires one shot at a time, fails to cycle IOW. It will eject but apparently doesn't come back far enough to pick up the next rd to feed.

He bought a collapsible stock & tried that, same result.

BUT: if he puts the spring & shock buff from the collapsible stock in the A2 buttstock, the GD thing works fine!!!!

My guess is the short barrel & gas system isn't enough pressure to send the bolt carrier back far enough with the full spring on the A2 stock but it also won't work in the collapsible stock which probably has the same/equal pressure.

What's the solution to this dilemma? He wants the collapsible stock for his wife to use. Does he go with a different recoil spring? Does he cut off a coil or two until it works? What?

I've checked all the obvious problems with the gas tube being pinched, gas rings on bolt, etc. etc.

Thanks in advance.

Is he using reloads?
GG

Try just swapping the buffer. IIRC the carbines use a different one than the 20" barrels. Check out this Colt FAQ page for buffer comparison.
www.biggerhammer.net/ar15/buffers/

And this Colt parts page for part numbers.
www.colt.com/colt/html/a4c16_buttstockassy.html

Quoted: Have you checked to see if the gas key was loose?

Yes. I checked the gas rings to make sure they're spread apart as per their gap. There's nothing physically noticeable wrong with the rifle. That's what bugging the crap out of me (us).

Originally Posted By Gun Guru: Is he using reloads? GG

No. We've tried using a variety of ammo, mil-surp as well as PMC, etc. All show the same exact symptoms.

Well you either need to fix the gas leak or open the gas hole up a bit under the front site.
Using the carbine buffer/spring is baaaad idea as the buffer is too short to bottom out on the inside of the buffer tube and there is nothing to stop the bolt carrier from hitting the receiver with the carrier key and breaking out the back of the lower receiver.

Now he put the collapsing stock on a pre-ban Bushmaster receiver right?

Quoted: Well you either need to fix the gas leak or open the gas hole up a bit under the front site.

How do we know there's a gas leak? The tube appears to be fine, no kinks or dents visible.

Keep in mind that the upper is a M4-type & the lower is the full size A2 version. I'm wondering of there's enough gas in the system to work the bolt back?