7mm is an excellent choice for elk hunting. As noted above, all these use the same
bullets (diameter = .284"), but each is a different cartridge case. The REAL reason for all these different cartridges is so the gun companies can try to sell you on the latest/greatest each year or two, and thus sell more rifles.

Stick with what works. Recommend you look at the 280 Remington, but if you want a little more power, go for the 7mm Remington Mag. Both are top notch for elk. You'll find a little better selection of rifles from the various manufacturers for the 7mm Rem. magnum. Personally, I see no use for magnums unless I'm stepping up to something
in the 338 or larger category, but to each his own.