I decided to try a slow powder in my 458win mag.I loaded 62 grainsof surplus IMR 7383 powder[using 4064 data as a guide] and to my surprise was very accurate.the gun shot a one hole group at 50yards and a clover leaf group at 100 yards.The bullet was a 350 grain hornady.Does anyone have any idea as to the velocity of such a load? I do not have access to a chronograph.