I don't know about GMX bullets, but Barnes monolithic bullets,
especially the early X bullets, shot patterns, not groups,
until one found the sweet spot for seating depth. The
first thing to note, is these types of bullets like to
"jump" .030"-060" to the lands. Seating close to the
lands had them, for the most part, very inconsistent.
I was about where you were with a 300 WSM, and a 150 gr.
bullets, seated .010" off the lands. I talked to Barnes
tech support, and they said seat them .050" +/- .01 off
the lands, and re-shoot...My 3" groups went to just under
1"! Since then, I have loaded Barnes bullets in about 20
rifles, and bolt action pistols, the worst shooting firearm,
shooting Barnes bullets was 1.5 MOA, all seated .030"-060"
off the lands. So check your seating depth, and your
chamber, and get some "jump" between the bullet, and the
lands.
Squeeze