I have a custom Switch Barrel rifle on a Win Model 70 Classic Stainless action in,270 Weatherby,7mm Weatherby,300 Win Mag and 338 Win Mag.
Out of all these barrels (all 26") the 270 is the most accurate by a fair margin.
The best bullet for long range (up to 600 yards) is the Nosler 140gr Accubond,in my rifle,I have tried others,but this performs the best.
I can get .25" groups @ 200 if I do my part with this cartridge.
You need to try different brands until you find the one your rifle likes.