What type of barrel, barrel length, gas tube length, buffer and action (buffer) spring is on the rifle?  

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Bolt ride over, and either the rifle is short stroking from fouling build up/ gas block coming loose when hot, or the mag is not being retained high enough in the mag well/mag not recovering the top round fast enough.

Also, if you are using the mag as a hand hold point, don't!!!  This can cause you to cant the mag in the well to dip the back side of the mag to allow the bolt to ride of the back of the rim to cause the problem you have.

So on the fouling part, make sure that you are cleaning the chamber with a chamber brush and CLP.

As for mag, make sure that the mag catch threaded section is flush with the face of the mag release button, and might help to pull the mags apart to CLP clean then as well.

As for the gas block, double check to make sure that the set screws are tight, and that they have been loctite in place.  Also, with a B/C swap, pull the bolt off the carrier and dry fit the carrier with key into the upper receiver to confirm that the gas tube is correctly indexed with the new carrier key.

Out of curiosity, have you tried a different weight buffer or a heavier resistance action spring (buffer spring), to see if things change?

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Lets make life simple and just weed out the mag retention in the rig's well to begin with.

Strip a mag down to just the body.  
lock the bolt home, and lock the mag in the well via the catch.
Now take a pencil's eraser side, shove the mag release button all the way in, Shove the mag all the way up until you bottom the top of the feed lips out against the bottom of the carrier, take the pencil and mark a line on the back body at the bottom of the well entry lip, then let the mag slip back down to lock back on the catch.

What is the distance from the mark line on the mag back to the bottom of the mag well, hence how far down is the mag sagging in the well locked by the mag catch from the bottom of the carrier.   Ideally, should be around 1/16", and still good up to around 1/8".  If you are into the 1/4" range plus, then here is the problem, since any rocking of the mag during recoil will allow the back of the mag to dip down, and will allow the bolt to over run the top round in the mag.


So once we can weed out the mag being retained too low in the well as the problem or not, then it comes down to either the mag not recovering correctly, or the bolt not coming back all the way instead.

If the bolt is locking back on the bolt catch after the last round fired, then that weeds out the rifle short stroking, and just leaves the mag not recovering correctly instead.  As for mag not recovering correctly, first solution here to pull the mag down and CLP clean it.  Hence fouling can build up inside the (even when new) and cleaning the mag parts often resolves this problem of the follower binding up inside the mag body.