I use WC860 in mine. If you look in the link that I posted, Kirby gives powder weights and velocities. You can't just "fill up the case", you've got to develope a load just like any other cartridge, although there are some good suggestions on where to start on here.
Is this your first LR rifle? Are you an avid reloader? I would ask myself these 2 ?'s before diving into a AM. The 7 am is an amazing round but not a cartridge to a beginner. It was mentioned that you can't just go and buy cartridges for this caliber. That's just the beginning. You can't even buy brass that is formed for that caliber. You have to Buy .338 lapua brass, neck it down to 7mm and then fireform it to your chamber. Wait a minute I stand corrected: You can but the stuff directly from Kirby already fireformed but I can't imagine it being really affordabe.
I supplied the .338 brass to Kirby and he fireformed it for me. I didn't think the price that he charged for fireforming was out of line, and it was easier for me to do that, expecially considering that I didn't have the rifle and hunting season was fast approaching. I have enough fireformed brass to last for the life of the barrel.