An assault gun is a gun or howitzer mounted on a motor vehicle or armoured chassis, designed for use in the direct fire role in support of infantry when attacking other infantry or fortified positions. The term is a literal translation of the German word Sturmgeschütz. Nazi Germany introduced the first purpose-built assault gun, the StuG III, in 1940, thus establishing this category of armoured vehicles.
Historically, the custom-built fully armoured assault guns usually mounted the gun or howitzer in a fully enclosed casemate on a tank chassis. The use of a casemate instead of a turret limited these weapons' field of fire, but allowed a larger gun to be fitted relative to the chassis, more armour to be fitted for the same weight, and provided a cheaper construction. In most cases, these turretless vehicles also presented a lower profile as a target for the enemy.
The assault gun looks and works in the same way as the similar tank destroyer, the only difference in most cases being the gun. Assault guns generally used larger calibre, lower velocity guns, with their primary ammunition being that of high-explosive shells; these were meant for taking out soft targets as outlined in its infantry support role. This was contrasted with the tank destroyer, which utilized higher velocity, and therefore smaller calibre guns, firing armour-piercing shells as their primary ammunition. Therefore, these vehicles often sacrificed being able to fire a good high-explosive shell in exchange for maximal armour penetration characteristics. Towards the beginning of the war, a single vehicle could generally be used in both roles, but that changed as the classes became increasingly specialized as the war progressed.
Assault guns were primarily used during World War II by the forces of Nazi Germany and the Soviet Union. Early in the war, the Germans began to create makeshift assault guns by mounting their infantry support weapons on the bed of a truck or on obsolete tanks with the turret removed. Later in the war, both the Germans and the Soviets introduced fully armoured purpose-built assault guns into their arsenals.
Early on, the Soviets built the KV-2, a variant of the KV-1 heavy tank with a short-barrelled 152 mm howitzer mounted in an oversized turret. This was not a success in battle, and was replaced with a very successful series of turretless assault guns: the SU-76, SU-122, and the heavy SU-152, which were followed by the ISU-122 and ISU-152 on the new IS heavy tank chassis.
The primary German assault gun was the Sturmgeschütz III (StuG III). Late production StuG III variants, armed with a high-velocity dual-purpose 75mm gun blurred the line between assault guns and tank destroyers and were the Wehrmacht's most-produced armoured fighting vehicle, at some 9,400 examples. The Germans also built a number of other fully armoured turretless assault guns, including the StuG IV, Brummbär and Sturmtiger. The latter two were very heavy vehicles, and were built only in small quantities.
Battalions of assault guns, usually StuG IIIs, commonly replaced the intended panzer battalion in the German panzergrenadier divisions due to the chronic shortage of tanks, and were sometimes used as makeshifts even in the panzer divisions. Independent battalions were also deployed as "stiffeners" for infantry divisions, and the StuG III's anti-tank capabilities bolstered dwindling tank numbers on the Eastern and Western fronts.
US and UK forces also deployed vehicles designed for a close support role, but these were conventional tanks whose only significant modification was the replacement of the main gun with a howitzer. Two versions of the American Sherman tank were armed with the M4 105 mm howitzer, the M4(105) and the M4A3(105); these were designated assault guns in US usage of the term. The Churchill, Centaur and Cromwell tanks were all produced in versions armed with 95 mm howitzers: the Churchill Mark V and Mark VIII, the Centaur Mark IV and the Cromwell Mark VI. Earlier British tanks, such as the Crusader cruiser tank and the Matilda II Infantry tank were produced in versions armed with the 3-inch howitzer; the first versions of the Churchill tank also had this gun in a hull mounting. American tank destroyer units were often used in the assault gun role for infantry support.
The AVRE version of the Churchill Tank was armed with a Spigot mortar that fired a 40 lb (18 kg) HE-filled projectile (nicknamed the Flying Dustbin) 150 yards (140 m). Its task was to attack fortified positions such as bunkers at close range (see Hobart's Funnies).
In the post-WWII era, vehicles fitting into an "assault gun" category were developed as a light-weight, air-deployable, direct fire weapon for use with airborne troops. Current weapons were either based on jeeps or small tracked vehicles and the airborne troops thus always fought at a distinct disadvantage in terms of heavy weapons. The Soviet Union and the United States were the most attracted to the idea of providing this capability to traditionally light airborne forces. Their answers to the problem were similar, with the United States developing the M56 Scorpion and the Soviet Union developing the ASU-57, both essentially air-droppable light anti-tank guns.
The Soviets went on to develop an improved air-droppable assault gun, the ASU-85, which served through the 1980s, while their SU-100 remained in service with Communist countries, including Vietnam and Cuba, years after World War II. The US M56 and another armoured vehicle, the M50 Ontos, were to be the last of the more traditional assault guns in US service. Improvised arrangements such as M113 personnel carriers with recoilless rifles were quickly replaced by missile carrier vehicles in the anti-tank role.
The only vehicle with the qualities of an assault gun to be fielded after the removal of the M50 and M56 from service within the US military was the M551 Sheridan. The Sheridan's gun was a low-velocity weapon suitable in the assault role, but with the addition of the Shillelagh missile could double in the anti-tank role as well. The Sheridan, however, was not developed as an assault gun but as a light reconnaissance vehicle.
Currently there appears to be a move toward wheeled vehicles fitting a "tank destroyer" or "assault gun" role, such as the M1128 Mobile Gun System of the US Army, the Centauro Wheeled Tank Destroyer of the Italian and Spanish Armies, the Chinese anti-tank gun PTL-02 and ZBL08 assault gun, and the French AMX 10 RC heavy armoured car. While these vehicles might be useful in a direct fire role, none were developed with this specifically in mind, reminiscent of the use of tank destroyers by the US military in the assault gun role during World War II.