I've used Nosler Ballistic Tip 120grn bullets with 70 grns of Retumbo and 130 grn AccuBond bullets and 69 grns of Retumbo.
I chrono'd the 120 @ 3265 FPS and the 130 @ 3300 FPS. I think that the 120 could go faster, but will more likely just stay with the AccuBond as the BT's I had were the old solid base style, 100 ct boxes and they kinda frag pretty badly when they hit bone. The IMR 4350 load a half or full grain below max in the Nosler 6 manual give slower velocities, but very uniform velocity, about a 10 FPS spread over 5 rounds. Good luck with the 264, it's a very potent round.