I'm using in my 375X284 a 265 LEAD Gascheck Bullet for Plinking. Try the H870. A VERY slow burning powder.

My Full load is with a 300Grain Win Silvertip!! (When Win stopt selling bullets I went to all the shops and bought all the 300Gr Silvertip they had. Even when I went into different towns, was I looking for Gunshops) The 300 Silvertip has two canelures and I use the bottom one, since the bullet goes flush with the neck and I gain some more room for powder. The chamber has a long throat and the bullet is just touching the lands.