A full mag will have enough room in the mag for just about an extra round when loaded correctly.

If you have anti tilt carriers, and are packing the mag beyond tight (read trying to stuff 30 rounds in such), then the problem is not out of the norm.

Check to see if the carrier is still tight in the upper (key to upper slot, with the upper shotgunned open, the muzzle down 45*, and the pull back B/C allowed to free drop), and if there is an end bur on the buffer spring winds that may be scrapping down the tube.   Need details on the carrier and hammer  as well, since if you have an un-shrouded carrier, and a L cut hammer, the hammer may be catching on the FP collar on closure.


Also, if you have another rifle with the same type of butt stock, swap springs to weed out if the spring in the rifle is on the weaker side to begin with.

Lastly, try to match book trick (folded piece wedged behind the mag and well to tilt the forward upward to see if that helps as well.  Could be the mag sitting to low during the feed, and the case wedging between the mag lips and the barrel extension on feed.