I like the .308 for pentrating body armor. Hornady made a tungsten core round for DoD that is sweet as all get out. You can kill an engine block with it too!
Yep. I would personally prefer the M-14 over any of these: superior brush-busting power.
Design a sabot cartridge for special uses, as with the .30-06. (Maybe there is such a one; I don’t know.)