Back in the late 1950s and early 1960s when the .264 came out there were not as many bullets and powders as today. 870 and MRP was the go to powder for the .264 Win Mag. I just built a .264 Win Mag for a customer. Borden Timberline action, McMillian BDL in Edge Technology, with a 26", #4 fluted, Muller Works, 1:877, 5R, barrel. I shot it with 130 Nosler Accubonds, 130 Sciroccos, 140 Barnes XXX, and 120 Barnes XXXMX. I tried IMR 7828, RE-22, H-4350, H-1000. The 120 Barnes XXXMX with Federal 215, and 68.gr IMR 7828 shot multiple 5 shot groups in the .3s" with a velocity of 3,420. This is a hot load and I would not recommend it as a starting load.
Nat Lambeth