I have a 300 RUM that I was thinking of screwing a 416 barrel onto and chambering it to the 300 case dimensions. That way I would have a cape buffalo rifle (I know, it's not CRF) but could change it back to 300 in the future. Any thoughts?

Blacktailer

If by going back to 300, you mean using the original barrel then you might just have the most free floating barrel the world has ever seen.

Actually, you have a nice switch barrel with about tghe contour Weatherby use on their Accumark, that is about .72" at the muzzle. Enough for 416 and weight should be OK in 300 id the barrel is fluted.

On a Rem 700 you would not need a recoil lug on the 416 barrel and Rem 700s shoot best with no barrel bedding so it could bea real neat switch barrel gun.

Just buy 375 Ultra brass and either neck up or neck down

Mike

I've seen M70 (CRF) 300 RUM's going for a song. Get one of those and have your way with it. You'll be happier with the action, and the fact that you don't have to deal with switching barrels.
You'll spend $500 doing what you want to it (dies and all), so throw in another $400 for a proper gun!