RE: 6mm Remington and elk
Back in '78, we had just moved to Montana. My dad went out on our first hunt and killed a 353 net 6x6 bull - with one shot through the lungs from a 6 mm. The round will work.
You do have to make significant compromises on shot selection - avoid paunch and shoulder bones and long range - but in a pinch the round will kill an elk. (By the way, these are the same compromises that bowhunters make, so I laugh when I hear it can't be done.) The bullet will almost certainly not pass through an elk, so your blood trails could be sketchy.
That said, more gun is probably preferable, unless you have a truly pressing reason for going with that caliber.