Would help to know what butt stock is in play, since you could be running a longer A-2 butt stock bolt with an A-1 stock, and the end of the bolt is now protruding in the back of the tube void to wedge into the center of the buffer bumper.

Also, could be that the new spring has end of spring coil burs that are binding on the inside of the tube and buffer, and may need to take a file to the end of springs winds to remove the burs. Also, check the buffer bumper roll pin, and if protruding out the sides of the buffer, use a file to flush the roll pin with the sides of the buffer.


Last thing to check, could be that the C bottom grove in the new carrier is shallower than the old carrier, and its the tip of the buffer retaining pin that is hanging up the carrier instead.


Have you pulled the spring and buffer to take a look at them and in the side of the tube yet, and what else has been changed on the rig since it run correctly?

Inside of the receiver extension should feel slightly ridge from the way it lathed, but the question comes in did you install the end of the buffer tube so it over the outer lip and just about kissing the center post of the buffer retaining pin.

Also, Is the receiver extension something new you added with the new butt stock, or did you just swap the old butt stock to the new MagPul CTR butt stock on the same tube.  The reason that I ask, some of the cheap knock off receiver extension are problem children, and one of such can be the tube not threaded in line itself with the inside of the tube, causing the tube to be canted off to one side of the upper/lower receiver line.  With this, when the B/C is retraced back, the B/C get wedges in the tube instead.


As for tip of buffer retaining pin binding on the bottom U slot of the carrier, easy enough to pick up, and the tip of the buffer retainer pin will have a wear mark on it.

As for your photo, not showing up as stated above.  So on that note, double check the link to make sure that it correct, and may even have to find another hosting service for the photo's to post here if your link is correct.

Easy fix,

Top of the hammer firing pin contact pad  sharp edge needs to be more rounded, and the bottom of the carrier needs to be polished a touch  as well.

Simply, the hammer's sharp edge at the top of the firing pin pad  is catching on the bottom of the slightly rough carrier when the carrier is back, and what is holding the B/C back instead.

The lines on the bottom of the carrier are the tell tale binding signs of this problem.