 All prototypes had muzzle brakes and lengthened chambers for a new experimental shell (7.1 kg, muzzle velocity 710 m/s, range up to 14,060 m).Factory trials started on 8 May; on 9 June prototypes were brought to the Sofrinsky firing ground near Moscow.On 14 June the gun, along with other artillery pieces, was demonstrated to the country leaders including Joseph Stalin.