I shot 1000yds with every thing from 5.56 to a 300 WinMag. It all depends on what you want/afford. 5.56 you'll burn barrels and your brass won't be reusable. The 300WM barrel will last you about 1500 rounds the brass is usable, but the recoil can be troublsome. I've built a wildcat which is the 6x284. It is a 6mm/243 bullet weighing 107 grams and a 284 case necked down to 6mm. It travels about 3700 ft persecond and the barrel ilfe is the same as a 300WM with out the recoil.
Whoa!!! Paul, I need to hang out with you way more often! I wanna shoot that unique beast!