We need to define long distance target shooting just a bit here to give a proper response. If you're talking about 1,000 yard prone shooting, the 6.5/284 rules the roost in that game, hands down. the 1,000 yard bench shooters may go with something bigger, such as one of the various 300 Mags, while more recoil sensitive shooters will go with smaller, high velocity rounds like the 6mm BR, 6mm XC, or similar rounds. If you go with a 6mm, 6.5mm or .30 cal, you'll have a wider variety of true Match bullets available to choose from. A somewhat smaller selection in the 7mm bore, and downright slim pickings in .257", 270" and others which don't see much match use.
Hope this helps,