Well I was in a Texas Varmint Hunter shoot once in New Braunsfels Tx. in mid July. 15 targets 35 yards to 550 meters. There was 12 or 13 shooters up on the line at a time, 5 shots in 3 minutes I think it was. I was factory varmint Ruger M2 varmint 22-250 but every caliber known to man was on the line. The guy next to me was 30 cal big case import something with a brake and knocked my hat off at 15 feet, I was doubled plugged pretty quick. I was fortunate to have a man named Doc Uzarski for my spotter he used to shoot for the army and was way better at reading the conditions than I deserved. In Texas reading mirage is key to hitting a garbage can at 550 much less a prairie dog in the head printed on legal size paper at those distances. Here when mirage is running fast left to right you can find yourself bagged 3' off,, 200 yards can become difficult. Anyways I felt like a hobo but wound up 3rd with 1,2,&3rd being separated by 1 point As for caliber for a long range ,build whats appropriate for your skill level and conditions. There's a place near here where you can go shoot mountain top to mountain top for $50 usd with gongs up to a mile. Most of those guys are 6.5 and up from there. It always boils down to skill. I would be good experience to go to a timed contest and see first hand whats being used, take a rifle and your gear and try it.