Quoted:
Most of my data says that 850 - 925 yards is about the limit unless I am shooting in the Alps or Death Valley in the summer.  At most normal temps and conditions I go subsonic anywhere between 825 to 925 yards.  That's not to say that a 1000yd shot is undoable but the platform is probably going to be best from 800 yards in.   So far I have been able to average about 0.9 moa for multiple 5 shot groups.  0.75 being my best, 1.2 being my worst unless I just loose focus,  flinch or yank the trigger.  On a 500yd target my first outing, I dropped two sighting shots 2.5" apart, I fired 4 shot that were within 5" of each other.   3 shots of that landed within 1.5" of each other.  I was pretty damn happy.  Wind kicked my butt though.  Never fired past 100yds before that event.

Quoted:
Grunteled hit on the key, finding a load that stays supersonic out to 1000 yards. That will be very difficult with factory 308 ammo and an 18" barrel. A longer barrel improves muzzle velocity needed to stay supersonic longer. For example, Palma shooters use 30" barrels to stay supersonic at 1000 with 155 gr SMK bullets, but 30" barrels are unwieldy.  Increasing bullet weight also improves downrange velocity but really heavy bullets displace too much powder volume in the cartridge case and muzzle velocity suffers.

A potential solution is to use a smaller diameter bullet with high ballistic coefficient. 260 Remington and 6.5 Creedmoor are two cartridges with good reputations for accuracy at long ranges. If shooting out to 1000 is important to you, you might consider one of those cartridges and custom barrel by Lilja or another custom barrel maker.