While some rifles prefer different ammo to others, ammo loyalty is harder to budge than getting an Arsenal fan to go support Spurs.
I have used Eley subs for ten years and never considered anything else as they have proved perfect for me. Six years ago I met someone with exactly the same view of Winchesters and we tried 20 of each others ammo on paper. At 65 yards the grouping of each rifle moved by 3-4 inches, but the actual groups were the same size as before. Logically we both know we could change ammo without a problem, but neither of us did.