I've shot high 190s with more than ten Xs at 600yrds with my service rifle. Compaslake prepared Douglass XX 1/8 barrel, Wylde chamber, Sierra 80g, US mil pulldown case, BR4, 25.3 VVN140. The two shooters I know that were testing the little cartridge 6.Xs both moved up to .243s, the 'perfect cartridge'.
FWIW