I found an older forum post (http://www.longrangehunting.com/forums/f19/extended-magazine-box-browning-bolt-51901/). The WSM also has this piece of plastic, but not as large.
I have a Browning A bolt in 300WSM that I am trying a couple Barnes long range hunter loads, but I ran into an issue. I tried seating the 200 grain LRX bullet to .030 off the lands, but it is too long for the magazine. Does anyone know if there are aftermarket magazines that allow a longer...