pacnor sent me a small bottle of witches brew for break in, which I used as instructed on their breakin sheet that was also provided. Follow the break in procedure and do it right. don't get worried about groups with factory ammo.
If you complete the break in andtrouble shoot all the stuff already mentioned, then contact pacnor, they are very good folks there. I'm sure they will take care of you.
the duramax stock has an aluminum bedding block right? that should be all you need for bedding.
if you're limited to factory ammo, try some match ammo (link below) - hopefully you got a 1:8 twist?? if its 1:9 you will need 130gr or less bullets.
.260 Match Ammo Comparison Test