I guess first of all if you have been shooting cheap ammunition that has varnish on it your chamber is filled with varnish causing the cycle to be short.
If when you pull the charging handle back you feel a significant amount of resistance there may be debris in the trigger area. Or something is going on with the stock.
If it is a full length stock is the screw too long striking the buffer Nylon? If so it will leave a contact mark on the center of the nylon of the buffer. Is the Extension Tube Spring the correct one? (Buffer Tube)
Is it hard to insert a magazine with the bolt closed which could occur by a damaged magazine and it's lips.
But as stated a loose Carrier Key is in most cases the problem if the gun has been running good in the past! *Do Not Use Thread Locker! If it is loose take it off clean all areas and reinstall it. Then stake the screws! It is easier if you stake the Carrier Key Screws with a Center Punch coming straight down on it and in a triangle staking pattern around the screws. I will be glad to send you a picture of the locations.
On a very rare occasion a screw can break on the Carrier Key causing it to flex up and allow gas to escape when the weapon is fired. To check it just take a hex took and attempt to tighten the screw underneath the staking. If it clicks when you attempt to tighten it you have a broken screw. Once again remove, clean, use a new screw and stake it.
Lastly would you have happened to put a weaver style gas block on it and cut off part of your gas port in the barrel? Only a thought! Happens very often.
Check out the above and let us know!
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Last edited by Sniper03; 08-26-2014 at 09:13 PM.