Unless you reload or plan to pass on the RUM. Factory ammo doesn't realize the RUMS potential. Also the fact you haven't picked up a rifle for 5 years..I wouldn't suggest the first one being a big magnum. RUMS can be sharp and even the 300 wm or 300 wsm is at or beyond the limit of a lot of shooters in felt recoil. Not meaning to scare you, realize I have no info or idea on how you shoot/handle recoil just that its been years since you did, so giving food for thought.

Dang near any caliber with the right loads and time behind will work. The key is know your gun, load and limits. That makes up for inches down range everytime. If you want a magnum then i'd suggest looking at a 270wsm or 7mm rem mag. I lean on the latter and have had no problem taking game to 500+ yards.

Good luck