Keep it a 300 RUM, 338 EDGE which is a 300 RUM necked up to 338" with a long 28-30" bbl shooting a 300g SMK around 2900fps. You could do any of the various 270 or 7mm Allen Mags which are ballistically unmatched by any other caliber in there class. If I'm correct, you might also be able to do something based on the 338 Lapua or the 30-378, 33-378 cases. Someone correct me if I'm wrong on the above.