Logic suggests that peening with a spoon makes more sense than with a ball peen hammer. I reason this way: with a ball peen hammer there is a relatively sharp curve to the peen part of the hammer. Thus, it is more likely to glance off the pin and strike the adjacent scale. This in return would require a scale repair after peening. The striking surface of a spoon is much gentler and thus much less likely to glance off the pin and strike the adjacent scale.