The more I look at ballistic tables, the more convinced I become that the best mule deer cartridge is probably a .264 Winchester Magnum. It is a fairly easy shooting cartridge and with a 140gr bullet it has great energy and accuracy out to 500 yds.
There are dozens of other cartridges ballistically similar to this, but they all seem to have some trade off (more recoil, more expensive cartridges, etc).
