Dimitri. How hard are those lead bullets you are loading? When loading to those power levels, bullets should be as hard as you can get them. Some of the commercial bullets are swaged, and therefore too soft for high velocity loadings.
I load my own cast bullets, heat treated to a hardness level of around 20 BHN or harder, and leading is not a problem. Groups are about as tight as I can shoot. (Not as tight as I would like though) I'm talking 240 gr. Keith style SWC's at 1450 FPS using W-296. Also 300 gr. SWC's at 1300 FPS. This is in .44 Mag.
The important thing is, in my opinion, the bullet must be hard enough not to strip in the rifling.