About the only thing that the caliber has to do with the shot is making sure that it will get the bullet that far and keep it supersonic. People shoot 1k with everything from 22-250's , 6mmBR , 308Win , 30-06 and the such. If your gun is put together correctly with good parts then the rest is up to you.
Their are alot of FACTORY guns out their with FACTORY chambers shooting FACTORY ammo that will do a good job at 1k , I've seen Savage , Rem , Weatherby all shoot at 1000yds and be at 1moa with factory ammo.
If you give a $3000 Mike Bryant built bench gun with ammo it likes to a guy that has poor form , can't read wind or mirage or even if he has a flinch then he won't beat a seasoned bench or Tactical shooter with a good factory 308 and some BlackHills ammo.
Like sniper2 said , you can't buy small groups.