Love it when someone asks about taking a long range shot in a hunting situation on a long range hunting site and folks almost seem like they are trying to talk him down.
Now to the OP: the reason they are trying to talk you out of it is because of your rookie question, here's what it sounds like from the receiving end of that question: "Should I use a ballistically inferior cartridge to shoot at large heavily constructed animals at extreme long ranges?"
Now to really get things going, I wouldn't want either of those. I try to plan worst case scenario, and if I were to intentionally set up for a 1200 yard shot on an elk I would want something more substantial...like my .375 Snipetac and an appropriate bullet. The .338's pushing the 300's at a decent speed (as mentioned by a few) would be my minimum. Will a 7mm of sort kill an elk at that range, sure it will but you are pushing it (my opinion of course). One of your biggest assets at those ranges and on live targets is gonna be a spotter, and a darn good one. You ever try to see trace at those distances (and with a little 7mm bullet), or spot impacts on fur or in vegetation (and with a little 7mm bullet)? Your quest is doable, and it sounds like you have a solid year to execute. Good luck!
Let's recap:
I reminded everyone its a LR hunting site, in a nice way told the OP it wasn't a very good question, probably stirred up a few 7mm fans, and said a minimum of .338 was need on elk for longer ranges. Did I miss anyone?
First one to say anything about the 6.5CM gets ignored.
Now that I've poked just about everyone I think I'll go wake the wife up prematurely so that this day really gets started in jerk fashion.