The reason to avoid real long barrels isn't the weight, as some suggest, they aren't much heavier. The drag with them is if you're going to spend hours walking through the timber and brush -- 26+ they really start snagging on stuff much more than 26 and under (Colorado native and done it 40 years). It can be done, it's just a bit of a hassle. Ideally you can have 2 guns and if mostly walking to a long range shooting position, use a 30" if the cartridge and slow powders can get you extra speed. Then use something 20-25" for the mixed hunting including timber etc. If one do-all gun in magnum cartridge, 25/26" is a pretty good compromise.
Once you start going under 24", you really don't need a magnum cartridge, since you'll typically get nearly the same velocity from a non-magnum chambering. Or.. you might get a wee bit more speed with a bunch more powder, most of which is expelled as extra muzzle blast. In other words, a 20" 7mm mag seems silly compared to using a non-mag that can get you about the same speed with a filled case and less muzzle blast. Just my personal observations and what I practice after a lot a mountain mileage logged. Ymmv
