upper built, remove the charging handle, and the bolt from the carrier.

Dry fit the carrier with key into the upper to confirm that the key sides are not binding down the upper slot, and the gas tube is correctly indexed with the carrier key.
Note here, if you did not index the barrel nut correctly, hence top void spine or barrel nut channel dead center with the upper receiver gas block channel, and the gas tube can not slightly move in the upper receiver channel with pushed via a finger, then you will need to pull the barrel to correctly index the barrel nut with the upper receiver.


Now put the charging handle back into play, and will with your finger in the back of the carrier with key only, confirm that the key is not binding in the U slot of the charging handle, the gas tube is centered in the charging handle end opening, and when the key goes to slip through the charging handle end hole, it too is not binding.

If you are still good, then keep in mind that the charging handle does not pull back on the key nose to unlock the bolt, but pulls back on the ledge just in front of the key on the top of the carrier instead.   So still with your finger in the carrier pushing it forward, pull back on the charging handle to see if the binding is still going on.  Hence the front bottom  ledge of the charging handle is too low and this is causing the carrier to bind up on the bottom of the carrier passage in the upper receiver instead.


If still no problem, then take a very good look at the top and bottom barrel extension lugs, and make sure that the top lug is centered against the upper receiver gas channel, and the bottom lug between the two feed ramps is centered TBC on the front take down lug.  It's possable to cant the barrel extension in the upper receiver if too much torque is used to tighten the barrel nut.  Hence with the barrel extension canted in the upper barrel socket, this may not allow the bolt to cleanly release from the lugs when the bolt is cam'd to the unlock position forward.


Lastly, install the bolt in the carrier with the cam, push the bolt all the way inwards, then flick the carrier to see if you can get the bolt to move all the way forward that way.

If we are still good, then remove the charging handle, and with the B/C put back together start the tests again.