Single round loaded in a mag, insert the mag in the well, charge the round leaving the empty mag in place, then fire for effect.

The correct effect is the bolt should lock back on the bolt catch.


If the bolt does not lock back, then start checking for gas leaks/blockage between the gas tube to FSB, FSB to barrel, and key to carrier.

Also, bring me up to speed on the T1 buffer, and are you shooting heaver than 55 grain brass case ammo to justify using the heaver buffer.  

If the bolt is locking back correctly, the the cycle is fine and we need to see why the bolt is not coming all the way back forward instead.


Since I going to guess that the mags work in the other rifle, it a problem specific to this rifle alone.

Start by pulling the bolt off the Carrier and with just the carrier and Key, dry fit it into the upper receiver to check the alignment of the gas tube to the carrier key.  If you get any binding as the two meet up, the gas tube needs to be index tweaked to align it with the carrier key.

Next, take a look at the threaded section of the mag catch, and make sure that the tip of the threaded section of the catch assembly is flush with the face of the mag release button.  If it is not, then shove the bottom all the way in with a pencil, and reach around the other side of the receiver to thread the catch around another turn.


Moving along, take a look at the diameter of the FP stop collar against the ramp edge of the carrier (put the FP all the way forward).  If you have an L cut hammer and un-shrouded carrier, if the FP stop collar is protruding past the back edge of the carrier ramp, then the hammer can catch the lip of the FP stop collar to cause the B/C to hang up on loading. If you have this problem, then the FP cotter pin retainer will be bent to hell.

To continue, as you are checking the gas tube to carrier key fit, the carrier should be gliding in the upper receiver without any binding.   If you feel binding of the carrier traveling in the upper receiver before the key to gas tube engaugement, then determine if the problem is the key too wide for the upper (may have to lap the sides of the key), of if the forward assist prong is catching the carrier instead (FA may be installed incorrectly).

From there, need to know how sloppy the mag is in the mag well when locked into position.  Badly rocking from to back, you using the mag as a bipod, or even using it as grab hold, can cock the mag in the well in such a way to cause the bolt over/case binding problems.

Lastly, need a photo of your barrel feed ramps and inside of the receiver.  Could be that you have standard feed ramp barrel in a M-4 extended feed ramp upper receiver, or just nasty burs on the end of the feed ramps that is catching the bullet to cause the problems.