Several years ago sscoyote and myself dedicated 3 days to LR pd'ing just East of Pueblo,CO with specialty handguns.
The end result was my two longest kills were at 1590 yards and 1800 yards.
The 1590 yard kill was with a center-grip XP in 6mm-284 using the 115 grain DTAC (18" Broughton 5C 1-8 twist, MV 2725 fps).
The 1800 yard kill was a rear grip XP in 6.5-284 using the 140 A-Max (17" Krieger 1-8 Twist, MV 2775 fps).
We had some great conditions on parts of those days, not to mention some luck