I have a Bushmaster M4 upper with KKF A3 tac brake that short strokes. Edit to add, with A2 stock and buffer tube.
The problem has the following characteristics:
-Ammo independent. It short strokes with wolf, American Eagle, South African Surplus, and PMC.
-Magazine independent. It Short stroked across 8 magazines and several types, USGI 20s, USGI 30s, and Thermolds.
-Problem gets worse as the gun gets hotter. Barely noticible on first round, but gun will fail to lock back on first mag, and start having problems every round within 35-40 rounds
-Problem is independent of buffer spring. I had an old one that was at the bottom end of the recommended length and it short stroked with that one, it has a new one now and it short strokes with the new one.
I have changed the gas rings with no effect.
-The bolt carrier is key screws are staked and feel tight.
-The gas tube roll pin is present
When I insert my Colt sporter bolt, the bushmaster functions flawlessly. However when I put the bushmaster bolt in my colt, that gun also functions flawlessly. (Have not tested over the long haul, but it works over 60 rounds)
After reading the board, I will perform the CLP soak on the gas tube/barrel gas ports.
How? Just coat the front sight in CLP, shoot CLP down the gas tube?
Any other recommendations?
The key screws are loose, causing a gas leak when the carrier gets hot.
Since it may be that the screws have already stretched, may have been stretched at staking, or just bad from the start (heat treating), you need to get a new set of allen screws and reinstall the key.
Do not just retighten the old screws, or you take a good chance of having one or both snap off in the carrier, which you will need to dig out to replace (way fun).
The correct torque for the allen screws is 35 inch pounds. Also, when you stake them in, all is need is to flow a little of the allen head into the key metal. I have been seeing a trend where the keys are being staked too hard, which lowers the metal at the bottom of the key, and this in it's self is causing a leak (hand lapping required to remove the high spots) since now there is a high spot on the bottom flat.