In a 7RM, I would look for an 8 twist, that way you could shoot the 160 class bullets and the 180 berger, you would be surprised how well even the lighter bullets would shoot, you may lose 60-75 fps velocity over a 9 twist but the faster twist also helps with expansion and as long as the bullets jacket can stand the RPM's you will not over stabilize them.if your wanting to shoot the thin jacketed vld's I would opt for button rifling over cut rifled.
have a friend that I've been working with who does well with a 7 mag and 162 a-max's, I would go at least 28" on the barrel and prefer 30" if your not wanting a mountain rifle, keep it around 10# if you shoot off a bipod, preload the bipod, next to proper setting of the parralex it will help most to make your shooting consistant.
RR