Yep, as someone already said...standard length action, 300 WinMag. Short action...300 WSM. Good factory ammo for both is readily available.
Just curious, have you thought about getting into handloading, or having a friend who is into it load for you?? Getting 800 yd accuracy may be a lot easier if you have a load tuned to your rifle...Most of the (readily available) factory ammo designed for long range accuracy features match bullets...these won't be cheap and also may not be the best for choice elk...nor will higher quality hunting loads with the bullets you'll want to use for that kind of application, even though these are relatively common calibers.
Long term, look at the cost of a single-stage press and decent dies, and you'll see that it'll be more economical over time to roll your own! Get with someone who handloads, get a basic gear list and price it out (RCBS starter kits are a pretty decent value) and then check/compare ammo prices...$2-3 per round OR MORE for the good stuff with the chance that it won't perform in your launcher, versus say $600-800 initial investment for equipment that'll last a lifetime. Also, just add a set of dies for any other caliber you get into. I'm not not knocking your choice NOT to handload, but 800 yard hunting...well, there's a reason most of the people here are handloaders. By the time you buy 20 boxes of premium ammo, you'd have paid for the equipment and supplies for about the same number of handloads. After that it's a lot cheaper to load the high-end stuff and get what you're after.
