QuickLoad ballistics tells me that you need 2,800fps at the muzzle to stay supersonic at 1,200 yards.
Asking what loads/powders would give me that from 26" while staying at/under 62,000psi and 107% fill (beginning compression) give me this:
This is reasonable for the
theoretical answer.
Bart can give empirical data.
The rifle will tell you if its accurate enough.
DO NOT LOAD FROM THIS DATA DIRECTLY
Work up from lower loads and use a chronograph