Before carrying a defensive revolver, go to the range and shoot it at 25 yards double action in controlled pairs. You should be able to keep both rounds of every pair in the scoring rings of a B-27 target. Then, shoot the revolver without a rest at 100 yards. Practice this until you know just how much of the front sight you need to see over the rear to hit the target regularly (3 out of 6 rounds or better).
The above is predicated on the fact that you should be carrying a quality revolver that will shoot nice, tight groups from a rest. "It shoots better than I do!" You should practice until you shoot as well as it can.
CPT, Armor (Ret)