Looking for a flat shooting store bought cartridge for a 300Win Mag. Would like to take a deer at 300-400 years. Any ideas?

Hornady Heavy Magnum 180 grain BTSP loads.

Looking for a flat shooting store bought cartridge for a 300Win Mag. Would like to take a deer at 300-400 years. Any ideas?
If I had 300 years I would want to take more than one. If you can locate Hornady ammo loaded with their 165 BTSP bullet in .300 Win mag ....the only factory ammo that seemed to work well in any .300WM I have tried it in.

Federal Premium with 180 gr Nosler Partition.