Besides a .50 Cal. BMG is there a smaller caliber that is accurate at 1000 yds?
.30 cal is a standard sniper round, the .308 being standard NATO and the 30-06 (.30, '06) being more versatile; the two are the same cal. Both are accurate at 1,000 yards, but neither are very useful over 1,300 yards. The .50 is a FAR more powerful round.