Re: GOT an ACTION and BARREL NOW WHAT?
What contour is your barrel?
How heavy is your rifle going to be?
Have you shot either round?
I hate it when people answer my questions with a question, but in this case it might help you decide on which cartrisdge to choose.
Some guys are going to say "RUM all the way". But, if you're building a light carry rifle the RUM can cause flinching nightmares - it can be a handful without a brake on it and some of us don't like brakes on their hunting rigs for obvious reasons.
If your barrel profile is heavy and you're used to big rounds, then this flat shooting SOB may be for you.
The WM is no slouch. It will easily get you to 1000 yrds and knock down whatever you hit. The WM with a 200 gr Accubond is a lethal combination. In a light rifle, the WM is tolerable.
Billy Bob is going to say he shoots a box of shells through his RUM and doesn't feel a thing - good for him, I'm more accurate when I don't think about being punched in the shoulder by my rifle.
If you reload then this might be the one. Once you fire-form your brass the head spacing issue off of the belt is a non issue (a custom chamber make it mute anyway).
So, decide if it's going to he a carry gun or more of a heavier hunting long range shooter and shoot both cartridges before you decide.