+1 what CD says " I really think 120gr is the ideal weight for the 260rem. If you have them, try some faster powder's like Benchmark or RL15. Mine certainly seems to like a lighter charge of fast powder much better than a full case of slow stuff."
FWIW - I am using the Nosler recommended top load for RL15 + 120 NBT, in Nosler brass, which gives 2895fps from a 22" LW barrel 1:9 twist. Consistent 7fps overall spread on muzzle velocity, and .3MOA. All a bit academic I know, but it drops fallow and roe with no great meat damage.
Might be worth a try if you can scrounge a cup of RL15 to test.
Peter