Well gigantes failed hit on costello certainly showed that a .32 acp wasn't the best choice. But considering that most mob hits took place years ago when the revolver was still the dominant handgun, i would say a snub nosed .38 is probably the most likely answer. As for the most effective, a sawn off shotgun would be a good choice. Its somewhat concelable and a round of 12 gauge buckshot at close range is something you would be very lucky to survive.