If 800rd barrel life is acceptable, seems the best choice is the 6.5/284
The .595 Ballistic Coefficient of the 142gr matchking or bit higher 139 Lapua scenar, and maybe higher still #s for Berger bullets are gonna give you best trajectory potentials. Got that Lapua brass already formed, probably even get a Savage already configured and ready to go.
The remington 7mm SAUM is a similar option but offers a higher barrel life threshold from what I've read over on accurateshooter. Great article library over there for ctg discussion. Very good article by tactical gunsmith Terry Cross on his 6.5-08 Ackley rifle and its close coat-tailing to the performance of the 6.5/284. Cross uses R-P brass! Heresy and Sacrilege...
Then there is the 6mmCM the GA Precision pioneered ctg, the 6mmXC, which offer 1000yd performance with 107gr match bullets, but lower recoil and good barrel life. Unless you really want that 1mi potential; not so many reasons to choose the 6.5/284.
With a smaller bodied ctg like .22-250 or PPC/Grendel you have the better bullet-seating range plus short-action receiver potentials enabled. Possibly the best option for .478" boltface could be the 6mm or 6.5 Dasher, an Ackley-ized and extended case body variant of the 6mm BR.
Just for fun, here is a synopsis of the 2013 results of James O'Hara's 1000yd Dasher performance from accurateshooter:
"On 1000-yard benchrest targets, the 10-ring is just seven inches in diameter, while the X-Ring is a mere 3 inches in diameter. At the Virginia 1K Championships, James managed to keep all his shots within the seven-inch 10 Ring with eight of the shots inside the 3 inch X-Ring. That is amazing accuracy and consistency. David Goodridge says: “This is truly a remarkable example of superb marksmanship, rifle design, assembly, load development and equipment maintenance.” (O’Hara had previously set a 10-match Aggregate Light Gun World record of 4.5389″ in 2012). "
(
http://www.accurateshooter.com/competition/ohara-sets-1000-yard-ibs-agg-records/)
With the Dasher, you're looking at overall case length of about 1.6" leaving tons of room for bullet seating. In a repeater rifle, probably want a single stack magazine for most reliable feeding. The 107/115gr 6mm match bullets are very long...
8 shots out of 10 at 1000yds inside the 3" X-ring.... Pretty amazing performance! Why not build the rifle to suit the cartridge?