AR-15 short cycle problem

[firearmdoc1]

I am having trouble with the bolt not coming back far enough to feed rounds from the mag. It does eject. The bolt stop catches the bolt carrier, not the bolt after the last round. If I manually operate the bolt it comes back far enough to clear the bolt stop completely.
I am using an adjustable gas block, 18" barrel. I have switched bolt and carrier, buffer, spring and tube. Replaced gas tube and tried a different upper with the same results. Also replaced the bolt stop and tried different mags as well.
I am at a total loss so any suggestions are appreciated.

 

[M4Builder]

It seems to me (former armorer) that you need to try a new gas block.

 

[jscandale]

Assuming the cartridge is a .223, the things to check are; mechanical related issues, and gas related issues.

Mechanical related issues:
Check to make sure the action functions smoothly without any hang ups or any irregular resistance other than the hammer of buffer spring. One thing that is hard to detect and can certainly cause problems is the roll pin that holds the buffer bumper in place. Make sure that it is driven flush on both sides of the buffer. If it sticks out even a little, it will ride the coils of the buffer spring and slow the action just enough to cause your problem.
Make sure that the bolt is not carbon bound in the carrier.

Gas related issues:
Gas port size related to bullet weight powder burn rate combination is largely responsible for proper cycling. It would be good to know what kind of ammo you are buying/loading for the rifle and what gas system length is on the rifle (rifle, mid, carbine). Make sure that you gas block is properly mounted on the rifle so that the holes are properly lined up. Adjustable gas blocks are sometimes a cause for problems and are frequently used to compensate for an improper gas port size. Check to make sure the bolt tail is not excessively worn. A new bolt will measure about .2503". A bolt measuring under .2500" can sometimes be problematic. Make sure that your gas tube has an unworn bulb on the end and that it fits with minimal tolerance in the gas key. Also, make sure the gas key is tight on the carrier and that the hole from the gas key to the carrier body is not filled with gummy carbon.

Those are the major things to look for. Sometimes other things can happen, but this will cover most of it.

JS

 

[mac86951]

You say you have an adjustable gas block, do you have the adjuster screw too tight? Back it out a few turns, does that help?

-Mac

 

[Gun Smith]

What ammo are you using? Just a thought, but I've had the same problem with the Russian 5.56 steel cased stuff. It was underpowered in a couple of AR's I've tried it in.

 

[firearmdoc1]

Thanks for the replies, I got it up and running today. I had to turn the shoulder back. 030" that the gas block stops on due to mis-alignment of the ports in the barrel and gas block.

 

[M4Builder]

Did you turn that off the barrel or the gas block?

 

[firearmdoc1]

I turned it off the barrel

 

[jonbearman]

Firearmdoc,That was exactly what I was gonna suggest.I have seen it quite a few times on my own rifles and others.