Use the same powders you'd use in a longer barrel semi-auto. It is an old wives' tale that you need faster powders in a short barrel. If you want top velocity, use the "long barrel" powders. IMR4895 is a classic powder for mid-weight bullets in auto '06s. Slower powders may give higher velocities (Nosler and Hornady agree here) but the action may not operate correctly with the higher port pressures.