Shotgun upper open, stick you finger in the back of the carrier, and work the B/C all the way, all the way forward to check for binding.  It may the key is too wide for the upper slot, or the gas tube has not been correctly indexed with the key.

On the mag release threaded section end, make sure it has been threaded into the mag release button to be flush with the face of the mag release button.

ON the barrel extension, make sure it ramp type matches the upper receiver.  If you have a M-4 extended ramp upper in play with a standard non-ramped upper receiver, is going to cause feeding problems.

Pull the buffer spring and the buffer to check for both end of winds burs on the spring coils that may be scrapping down the inside of the tube/ face of the buffer, as well as checking the buffer bumper roll pin to make sure it not protruding out the side of the buffer to hand up on the recoil spring.

ON mags, the mag is fully loaded with you can push the top round down about 2/3 of the way of being able to load another round.  If a 30 round body has a anti-tilt carrier in play, it will become a 29 round mag instead.

Lastly, make sure to CLP lube the upper receiver surface bearing areas, including the buffer and spring before they are installed.
Note, void depth of the receiver extension dictates the buffer and spring lengths needed, not the type of upper receiver on the lower receiver.


Short of the above, could still be a problem with the hammer catching the firing pin retainer pin to slow the carrier down, or even having a Extractor tension extractor spring with a O ring installed around it, and too much tension for the extractor to be able to open up to climb over the live ammo rim during loading.

Really, show me a photo of where the bolt face is during lock up, and I can narrow the problem down to the specific item that is the problem.

First check is to shogun the upper open and via your finger in the back of the carrier, work the carrier by hand back and forth.  As stated, side of the key should not be binding on the sides of the upper track and the gas tube is aligned with the key.

Next, pull the B/C from the upper, and look inside the upper down the ejection side of the channel. Make sure that the pawl of the forward assist is not protruding into the void of the channel when the FA is at rest/sprung all the way out against it roll pin.

From here, pull the recoil spring and buffer, and make sure that the end of spring coils do not have burs, the roll pin on the buffer is not protruding out the sides to catch the coils of the spring, the spring and buffer are the correct lenght for the receiver extension void, and then lube and reinstall them.
To add here, if you have the receiver extension installed correctly, the the lip of the tube should be holding the Buffer retainer pin in place when you remove the buffer and spring.





If we are good so far, its one of two that we still need to solve.

So the first possibility is Hammer is catching the FP retainer pin as the carrier is moving back foward,
Shotgun the upper open, but will bank that you have not only a hammer that has a L cut top of contact pad, but FP stop collar is protruding past the ramp of the carrier.


Starting with the FP, Bolt forward in the carrier, shove the FP all the way forward, and make a mark on the face of the stop collar of the FP of how much of its lip is protruding past the ramp of the carrier.  Now pull the FP, chuck the tail of it up in a drill, and spin file down the outer edge of the FP stop collar until it edge is flush with the ramp of the carrier on a recheck.


Next if you do have a L cut hammer in play, we need to radius the top FP contact pad of the hammer so is not a sharp edge that will grab The FP collar to stop the B/C as it moving forward.   The hammer is very hard, but someone like a diamond file will make short work of such.
Note, on the hammer, remove it from the lower before you start filing on it.  The last thing you want is the file debris in the lower receiver void.

Note, the real problem (unless you have a M-16 FP in semi carrier), is the distance from the pin channel in the trigger, to the front edge of the sear, it too short (trigger out of spec).  This causes the hammer to not be retained low enough when locked back on the trigger sear to allow the semi FP collar to pass by the FP collar cleanly.  bBut since we can lengthen the trigger, we can mod the parts is catching on instead.

With these out of the way and the rig back together, the last thing we need to do is a slow pull and return on the charging handle.  The last test is to make sure that the top of the bolt catch is not too tall, and maybe catching the bottom of the carrier as well.  What helps as well, is when you are slow returning the CH with the B/C in tow, is to lightly push down on the front bolt to try to cam the carrier a touch more downward in the channel to make sure that the B/C is cleanly passing the Top of the bolt hold open in it downward position.

If you can feel the B/C catching the bolt catch, then the top of the bolt catch needs to be slightly lower. But before you start to file down the top edge of the bolt catch to get the needed clearance once it has been removed from the lower (again so you don't get filing debris in the receiver), double check the bolt catch channel in the lower receiver to make sure that the Bolt catch can drop all the way down to come to it normal home position.

Note, the photo is really about the Bolt catch tab not long enough to catch the follower tab (9mm mod'd catch solves that), but since you can see the tag marks on the face of the catch from the carrier tagging it in the down position, works just as well to show just how much of the top of the catch needs to be lowered.
9mm mod'd bolt catch is this becomes a problem with the mags not activating the bolt catch on a empty mag.
http://www.ar15.com/forums/t_3_66/479984_Problem-bolt-catch--help-----Fixed-01-29-2010--------.html

With these mods out of the way, lube both the inside and outside of the B/C with a good layer of CLP, install it in the upper receiver, do a few empty cycles of the B/C with the CH, and the problem should be solved.