...Only problem is that the 260 has that compact rifle barrel of 16 1/2 inches, and I don't have all the muzzle velocity I'd like to have for 300 yard shots...
Have you looked at the 100 gr X bullets and partitions? You should be able to get 3000 ft/sec from them in that 16 1/2 inch barrel and do fairly well out to 300 yards.
You'll only be about 7" low at 300 if you sight in at 200. Alternatively, if you sight in at 300 yards
, you'll be less than 5 inches off the line of sight out to about 350 yards.
Velocities will be high enough to reliably expand the bullets over those ranges as well.