I have a 300win mag and a 300wby mag. Would a 300RUM give me much benefit over the other 2? Or would it be better to just step up to the 338? Rifle will be used for hunting and a little target shooting.
My favorite rifle is my 300 RUM. Shooting Berger 230's at 3005. If your 300 WBY likes the heavy bullets I do not see much difference. The Berger reloading manual shows them at the same velocity. Go for it 338!
Having the .300 Wby in your arsenal, I would say you won't gain much by going with the RUM. If you want a little bit more out of your .300 Wby, you might want to look into the .300 Ackley Improved Magnum. It is basically an Ackley Improved version of a .300 Weatherby Mag. It just adds a little bit more oomph for your money, and forming brass is simplistic and very uncomplicated. You have your smith open the chamber up to .300 Ackley, load new .300 Wby brass as if you normally do, then shoot them. And out-pops fresh .300 Ackley brass. You will have to order a custom reamer and a custom set of dies...Which is the only draw-back. But other than that, there is no drawback.