7mm Rem Mag to .264 Winnie- No problems that I have encountered, I personally like to graphite my necks inside and out. I also use Imperial sizing die wax on the case body. Use your friends rifle as a test platform for the resized rounds. Just a slight resistance on the bolt when you close it on the resized round.
Leadman is very correct. The Factory ammo uses a 2 diameter J word bullet. If you use regular .264 Bullets , you will be seating them quite deep compared to the factory round.
I have been using surplus H870 and a 140 gr. Accubond in an early Model 700 with a stainless barrel . I found the bullet was cutting into my case space, reducing my powder capacity. I had my rifle throated by a local smith to get the bullet further out of the case. The .264 really spins coyotes good......