Don't know which is their weapon of preference in America, but in Italy their traditional weapon was initially the sawn-off shotgun and later the Kalashnikov machine gun.
What I don't understand is why the Italian and Italian-American mafia rarely uses sniper rifles, like for example the Russian mafia does. When they killed prosecutors and judges in the 80s and 90s in Italy, they preferred to blow them up with all the bodyguards using bombs instead of quietly do it with a single shot.