I have a Montana x3 also,I have used 270,3006 brass,through a 280 fl sizer,then fire form with 35 of h4198 works fine except wrong caliber marking.i think 280 Nosler brass to be best. Nosler black loading manual has a lot on 280ai,140 grain acubond i get3150 and 160 acubond I get about 3000 with h4831sc,almost 7 mag without the blast and recoil