Anything that is .240 caliber. Some fire better than others. Depending what your shooting at. Ask your local gun shop for help.

"The .240 Weatherby Magnum was introduced in 1968, and it was the last Weatherby caliber introduced while Roy Weatherby was alive. The .240 Weatherby case is unique. It is a standard length and diameter cartridge with a .473" rim diameter, the same as a 30-06... It is a very cute magnum cartridge... The .240 Weatherby is the highest velocity commercially produced 6mm cartridge. It will drive a 100 grain bullet about 150 fps faster than the 6mm Remington or .243 WSSM ... .240 Weatherby factory ammunition is offered only by Weatherby. Bullet weights of 87, 90, 95, and 100 grains are currently offered. The .240 case is unique and CANNOT be [fire]formed from any other existing case, which has somewhat limited the cartridge's popularity with reloaders." from www.chuckhawks.com/240weatherby.htm basically you have 3 options: 1) buy your ammo directly from weatherby... you're looking at between $50 and $80 *cough* for a 20 round box (not including shipping and handling... 2) buy from another supplier (read more expensive) 3)load your own (read just fractionally less expensive unless you place ANY value on your time) the round is basically a 30.06 cartridge that has been necked down to the .240 (6mm) caliber... this means all the power of a 30.06 behind a smaller (faster) bullet... reaches out and touches well beyond 400 yards... the greatest benefit of this round is the combination of POWER! and relatively low recoil.