After a carfull review in my books and notes the heaviest bullet I see is 300 grns .Those loads are for Blackhawk and Contender ONLY . My speer book shows 25000 cup 950 fps with AA#9 . Lyman shows a 295 cast bullet under 800fps No AA#9 . Maybe a 454 Casuall would be a better choice . The Colt has a working pressure of just 18,000 psi ,the 454 was created at 65,000 psi I think it has a SAMMI rating at 50,000 psi