A 16 inch middy isn't reduced in power. In fact, an over-sized gas port will make a 16inch middy grossly over gassed even with underpowered ammo. Federal XM193 55 gr isn't underpowered.
The rifle buffer and spring is the baseline using 5.56 spec ammo. If a rifle buffer is "too heavy" there is a problem with the gas drive- undersized gas port, leak or blockage in the system, under powered ammo.
UMC Yellow Box 223 might be under powered.
The usual causes for a bolt over base is a out of spec extractor or the BCG returning with too little speed.
An out of spec extractor will lose control of the spent case which will block the fresh round from feeding from the mag. Sometimes the spent case will exit the ejection port after it's done its dirty work. 90% of the time a new extractor spring is the right fix. The other 10% will be due to an out of spec extractor.
PRO TIP: Replace the BCM extractor spring now with a Colt or Sprinco M4 extractor spring. It'll save you a lot of trouble in the long run. No O ring.
A slow carrier will strip the round from the mag, but isn't fast enough to juggle it into the chamber. A slow carrier will bump the round out of the mag but without imparting enough speed to the round to prevent it from riding up nose high. With a nose high attitude, the tip of the bullet catches on the barrel extension forcing the base down. The bolt rides up over the rim and crushes the round, jamming up the works. Replace the action spring.
PRO TIP: Use a Sprinco green spring in the rifle RE assembly. Use either a Sprinco hot white or blue spring in the carbine RE assembly.
The last thing you need is a lighter spring. If the AR is short stroking with Federal XM193, there's a leak or blockage in the system.
The speed of the carrier would have to be really excessive and the spring in the mag very weak for the BCG to outrun the mag.
The H2 buffer is the right buffer. H1s work well, but the H2 is the standard. The carbine buffer is too light.
Perform a lock back check using the lower with the rifle buffer and Federal XM193 ammo. If it doesn't lock back, try another BCG that's known to work properly. If it locks back with a good BCG, replace the gas rings of your BCG and try again. If again it doesn't lock back, the gas key leaks or the gas tube at the receiver end is out of spec (worn or too small in diameter).
PRO TIP: The gas key screws can be properly torqued and still have a leaky key.
If the AR fails to lock back with a good BCG, there is a leak or blockage. The leak is either where the gas tube meets the gas key, where the gas key goes into the gas block or the gas block isn't sealing at the gas port. If there's a blockage, it's either in the gas tube, the gas block is misaligned with the gas port or the port is too small. I had one barrel where the gas port was mis-located and would not line up with the passage in the gas block- a neat trick consider how large the diameter of the passage is!
Do not worry about ejection patterns. Ejection patterns will tell you nothing useful about the gas drive. (It can be useful in identifying a failing extractor, however.)
Last edited:
