Depends on the Mob guy's objective. If it's an assassination of someone he's close to, and can put the gun right at the victim's head, a .22 revolver, firing CCI Mini-Mag rounds, is best--lightweight, concealable, little recoil or noise, no spent cartridges left behind, and CCI rimfire ammo is reliable. Two or three shots to the head would do it. For a frontal assault, nothing beats a tactical shotgun with a 18.5" barrel--don't have to aim it, just point it in the general direction of the victim(s) and down they go. For self-defense, a .45 ACP has maximum stopping power, and some compact and subcompact .45's are on the market. A 1911 is the best all-around self defense weapon, but it's not concealable.