I’m looking at building a big bore rifle for my trip to Alaska. But I don’t know what caliber would be best.
450socom or a 50beowolf?
If you're only considering those two calibers go .458 - unless the patents have expired the 50 Beowulf is a proprietary cartridge, which means everything about it including chamber dimensions is patented, privately owned and thus expensive [anybody wanting to use it must pay royalties on ammo, barrels, reamers, you get it]. But the .458 SOCCOM is a SAAMI standard cartridge with loaded ammo and components widely available. And it's just as easy to build your own AR in that caliber as any other. Or keep a spare upper in a Brown Bear caliber. [You'll also need a bolt, buffer and spring]
If you don't have your heart set on an AR then consider a lever action in either .45-70 or, if you're going to reload; the even better but much rarer .450 Marlin. The 450 was designed from the ground up by Marlin and Hornady to be a "Magnum .45-70" [check your Hornady manuals]. To get better performance from a .45 caliber rifle you have to step up to a .458 Winchester magnum. 450's come up regularly on Gun Broker, try to get one with the JM manufacturer's code [Marlin's USA plant designation]. Good luck.