Could be a lot of things, but pull your buffer and check the face of the buffer bumper to see if something has been tagging the center of it,

Hence looking at the butt stock in play, screams that the short A1 butt stock bolt should be used, and if your using the longer A-2 butt stock bolt (its 3/4" longer, the tip of the bolt can be protruding into the void of the receiver extension, and not allow your bolt to come all the way back.

A1 butt stock bolt on the top, so once the butt stock bolt starts to thread into the back of the receiver extension, is should lock the butt stock in place with less than 5  full turns of the bolt.


Hence full charging handle pull rearward, should have the front face of the bolt 1/8"~1/4" in front of the back edge of the ejection port window, and the CH pull from about 1/2 the way back, to all the way back, should be the same tension in the pull as well.

As for bolt locking back after the last round in the mag is shot, are you sure that the bolt catch is catching the bolt via the face of the bolt, and  not just the bottom of the carrier since the bolt can not make it all the way back behind the bolt catch instead?

Just a FYI on steel case ammo.  
You have to make sure to scrub the chamber clean by hand with CLP and a chamber brush every cleaning, since the poly coating on the cases get scrapped off to cause a powderized poly fouling that no standard cleaning solvent will dissolve to removed it that way (has to be scrubbed out of the chamber by hand). Also,  have to run the upper on the wetter CLP lubed  side to allow the CLP to flush out the powderized fouling before it gets a chance to collect and bind up in the chamber to begin with.

Not a fan of using the term "Horning", when polishing lapping an Ar chamber.

Hence if the chamber is reamer chattered with deep grooving, then if you try to hone such a chamber, will end up with the ID sidewalls demensions of the chamber to large and it going to end up trashing brass even from the brasses first firing.  Hence in case of a chamber with bad reamer job, barrel should be sent back for replacement instead.  Also to point out, if the chamber if chrome plated, a stone will end up chipping off spots of the plating in the chamber, making the chamber even worse.

As for lapping a chamber, this too is not really needed as well, since live fire will brass case ammo, and just using a brass chamber brush by hand during cleaning, will polish the chamber walls if they are less than a mirror finish.

Photo would help looking down the inside of the mag well with the upper shotgun open so we can see how much the mag catch claw is protruding into the lower receiver well area.

On some billet lowers, you need to mod the mag catch a touch so it can protruded into the well area a touch more (leave the lower receiver alone since you will just remove anodizing).

Hence deep of how far the mag catch claw will protrude into the lower receiver is decicated by the lower receiver milled area flat,

To add on this photo, this is the mag body alone in the mag well pushed tight against the bottom of the carrier, and in this photo, you can see that the mag is limiting out against the bottom of the carrier before the mag catch slot is height enough to lock into the mag catch correctly (bottom of mag side slot should be 1/8" above the bottom of receiver catch slot edge with the mag body pushed all the way up tight against the bottom of the lock home carrier).
So in the above photo, something is way out of spec, and the mag stands not chance of staying locked into the catch/well when bolt closed home.  Hence you may be able to lock the mag in the well with the bolt locked back, but when the carrier charges forward, it going to be trying to shove the mag out of the well/off the catch claw.

And when the lower receiver catch area flat is not milled deep enough, I will just chuck the catch up in a lathe to turn the area off the threaded rod down a few thousand as needed to allow the claw to protrude into the well the needed area.


Here, we have the opposite problem of the lower receiver flat begin milled too deep, since the flat to the left of the claw surface should be flush with the ID wall of the lower receiver wall . So in cases like this, will face the the indent area and raduis the entry of the claw to allow the mag to push the catch back when the mag is inserted into the  

To add here, when some mag wells are well over sized and you have a great deal of slop between the sides of the mag to ID sides of the well, you may have to end up with a touch of the center of the catch indent area protruding past the Mag well surface well.

Simply, if you find that something is out of spec and can get that part replaced, then that is the best course of action.  
But is such is not the option, then it's a mater of working the parts to get the rig into a working rig instead.  
So take it all the way back to if the mag catch slot was milled too high in  the lower receiver or something else out of spec to cause the problem in the first photo.  In cases like this and replacing the out of spec part is not a option, I will mill the portion of the top of mag catch tab/claw that protrudes into the mag well area to lower it, so it will allow the catch to lock into the mag and hold the mag in the correct relationship with the carrier above it (to prevent the binding problem when the carrier is locked home).

So again, parts changing is the easy way if that is an option if something is out of spec, but when parts swapping is not an option, then it's time to smith fit parts isntead.