liner in 1927 and a barrel 50 calibers long, becoming the 3-inch M1; a year later the M3 weapon was introduced with a larger removable liner. Another upgrade was started in 1931 as the T8, and then T9, but these projects were cancelled in 1938 when the 90 mm gun M1 was selected in their place. The 3-inch gun M1918 remained in service, particularly in National Guard units, and saw action in early World War II. In September 1940 a project started to adapt the 3-inch gun to the anti-tank role, starting with the T9 experimental model but equipping it with the breech, recoil system and carriage borrowed from the 105mm M2 howitzer. The gun was accepted for service as the 3-inch M5. A similar derivative of the T9 – the 3-inch M6 – was intended to be mounted on the M5 self-propelled gun, which was eventually abandoned. A final adaptation was the 3-inch M7, which included minor modifications for mounting on the M6 heavy tank and M10 tank destroyer. M7 saw wide use although it was supplanted to some extent by more powerful weapons such as the 90mm M3 and the British QF 17 pounder. 6,824 M7 guns were manufactured. Variants 3-inch Gun M1917 – Original fixed-place anti-aircraft gun introduced in 1917, barrel based on 3-inch gun M1903. M1918 – Mobile anti-aircraft gun using a barrel based on the 3-inch gun M1898 and a new mount. M1 – Variant of the M1918 with a removable barrel liner. M2 – Variant of the M1917 with a removable barrel liner. M3 – Development of the M1918 with a removable barrel liner, but on a mobile mount. M4 – Version of the M2 with a thicker liner for easier manufacturing. M5 – Version of the T9 adapted for anti-tank use. M6 – Version of the T9 as mounted in the 3-inch M5 Gun Motor Carriage. M7 – Version of the M5 for use on the M6 heavy tank and M10 tank destroyer Self-propelled mounts Heavy Tank M6 (M7 gun in mount T49). 3in Antiaircraft Gun Carriage T1 (Garford 7½ ton 6x4 truck chassis). 3in Gun Motor Carriage T1 / M5 (high speed tractor M2 chassis, M6 gun). 3in Gun Motor Carriage T7 (Trackless Tank chassis). 3in Gun Motor Carriage T15 (Ford 4x4 / 6x6 truck chassis). 3in Gun Motor Carriage T20 (Light Tank M3 chassis). 3in Gun Motor Carriage T24 (Medium Tank