Use Imperial Wax, and be careful to straighten all the necks with a 338 expander ball first, and they should form just fine. My 308-264 needed 2 steps but just barely. I do 416's, 560 in the body down to 340 in 4 very easy steps.
There's about 1760 yards in a mile. If you build one that won't shoot that far, I'd throw it away and start over!
Seriously, if you do a 32 or 34" barrel it'll have no problem with that range. At least not as much problem as you're gonna have with the cost of practice to get good enough with it.