An untraceable gun. A staggering majority of violent crimes involving firearms are committed using dumb gats. The mafia & criminals in general are not going to be deterred by gun-control laws, they will be willing and able to procure untraceable arms of their choice and use them to commit crimes irrespective of any laws.
Guido: "What you want I should do wit da gun once I shoot da poor bastard?"
Vinnie: "Drop the dumb gat and take the cannoli wit you."