Re: tactical rifle
An obvious question I missed...I assume your have a long-action...yes? (made that assumption given you were originally considering long-action magnums.)
Continuing with the assumption that your rifle is a long-action, building a short-action cartridge on a long-action is a great idea. By doing that, you will be able to seat your bullets pretty far out and still be able to cycle them.
In terms of cartridge choice, look at 7saum and 7wsm. Both are great cartridges. The wsm has a slightly larger case capacity and a result will shoot faster. A buddy of mine has a 7saum and he shoots the 180grain Berger Hybrid at 2900+....badass! Make sure you put an efficient brake on the barrel though as the both of these cartridges have considerable recoil that can make it difficult to spot your own shots without a brake.
I'm not gonna shoot here. I'm gonna shoot waaaaaaaay over there!