Re: ReBarreling for 1000 yards.
After a good friend broke the old 1000-yard scoped rifle record at the Nationals in 1970 with a borrowed 7mm Rem. Mag. and ammo, I've followed the use of different 7 Mags at long range events. They've had an interesting history. Especially when it was over a decade before consistantly good heavy 7mm match bullets were available.
28 inch barrels were the norm in competition rifles, not for accuracy but for a bit more muzzle velocity with 168-gr. match bullets.
Sight settings at 500 to 1000 feet elevation had to be 3 to 4 MOA higher that those used at 4000 to 5000 feet for 1000 yard targets. 'Tis the higher air density at lower elevations that slows the bullets down faster.
Powders producing best accuracy were IMR4350 and IMR4831 or other extruded ones in that range. While slower powders typically shot the same weight bullets out faster, those faster bullets didin't land as close together on paper a long ways down range.
As with any long range rifle and ammo intended for accuracy, don't get too enthralled about the smallest few-shot test groups you shoot. Pay attention to the largest ones; they're the size you can count on all the time with the load that shoots them. Whatever suite of components gives the best accuracy for a 15- or 20-shot string is the load to use. A good way to go about it is to decide what's the greatest distance you want to miss your point of aim at the target. If it's 4 inches at 1000 yards, then find a load that you can shoot into no worse than twice your max miss distance; 8 inches at 1000.