Try this load:

Caliber: 340 Wby. Mag.
Bullet: 275 gr.
Powder: Norma MRP
Charge: 85.0 grs. (NOT MAX LOAD BUT START 3 TO 4 grs. BELOW THIS CHARGE)
Velocity: 2751 f/s

I have loaded for my .340 Wby. Mag. for about 15years, and Norma MRP has given me the best acccuracy and velocity of all powders I have used.You can find Norma Powders from Black Hills Ammo.

Good luck.