Don't forget the 450 Bushmaster. It'll kill anything up to and including the big bears out to 200 yards and beyond. People say it uses handgun bullets only but that ain't true. The factory Hornadys use the FTX bullet which is a rifle bullet and there are others. This is a great round and hand loading for it is a breeze. Factory ammo is way cheaper and easer to get than the 458 or 50 also.
The 308 will not work on the AR15 platform, you have to move up to the AR10. I don't know much about the 300 Blackout, you'll have to Google that.