I did some LR shooting with the 204Ruger in a M21 Cooper with 40gr Vmax.It was petering out at 500 yards bad and if there was a slight change in wind I would miss a 11.5 inch wide steel plate.BC is king in LR.
I have shot my remington 700 in 204 out to 600 with factory hornady 40 gr vmax's on a calm day. would not try to shoot much more than paper at that distance though had some pretty impressive groups for that light bullet. Just got into reloading so now trying to find a load that works well.
I've killed numerous prairie dogs, badgers, and ravens with my GAP .204 out at 550/575yards in perfectly calm weather using factory Hornady 40gr V-Max ammo. Hopefully next time out with it I'll get her out to 600+
I was shooting 3" groups with little wind out to 450 with my stock rem sps in .204. I sold it however and went 223 so I could shoot something heavier at that range. Mine is for coyotes so I felt 75gr would be better. Accuracy was not a problem with the .204 at that distance though!
28gr of varget with the 39gr sierra blitzkings is all I feed my lrpv and it loves it but the praire dogs don't. I would however say 600yds is right on the edge of it's usefulness.