If you put it in that order (paper, deer, maybe elk someday...), then why do you want a magnum? Any magnum is WAY more power than you need for deer, and certainly punching paper, unless you frequently shoot from LONG range. The magnum will just burn a lot more powder to wreck a lot more meat, and it's not going to be the most pleasant thing to your shoulder at the range after a box of ammo or so. I've seen what a .30-06 will do to a deer, and I personally don't want to or think anyone really needs to do more with a high velocity mag. Your .308 is perfect deer medicine, and is just fine with the right bullet for elk at medium-long range (300 yards for sure, more if you place your shot right).

However, if you must build a magnum, my suggestion would depend if you handload. If you do, then I'd go with the .264 Win Mag. It's not going to beat the snot out of you, it shoots very flat with the high BC bullets typical of the 140gr+ 6.5mm bullets, and while the factory loads aren't real common and tend to be a little anemic, handloading will let you bring out the true potential of the cartridge (get a 26" barrel, 24" MINIMUM). If you don't handload, then I'd suggest the 7mm Rem Mag, simply because it's a great performer, and the ammo is everywhere, fairly inexpensive, and offered in a wide variety of bullets for just about any application.

To be perfectly up front. I'm not a huge fan of magnums, unless there is a genuine need for the extra power and/or velocity. They cost too much, kick too much, and are harder to shoot well without developing bad habits (i.e. flinching). But they do have their place, and some people shoot them well and even enjoy them. I, personally, would recommend something more along the line of a .280 Rem (Ackley Improved. if you handload). It'll do almost everything the 7mm Rem Mag will do, burning a lot less powder and with a lot less recoil. But that's just me.

Mike