The Manville gun, was the police response to the escalating war against the mafia and their use of the tommy gun. The Manville gun was not very well designed, and thus only used for a few years. Three of its main design flaws were a lack of stock (to assist with balance, aiming, and recoil), lack of trigger guard, and lack of a safety button. Owing to excessive collateral damage and injuries to innocent bystanders, the Manville was soon retired from police use.
Visit the Museum of the American Gangster to see the Manville gun on display.
Caption: Manville 26.5mm gas and flare guns holding 18 rounds.
Image from: http://smith-wessonforum.com/members/handejector-albums-fbi-jerry-campbell-picture5728-hoover-w-motor-traffic.jpg