An assault gun is a gun or howitzer mounted on a motor vehicle or armored chassis, designed for use in the direct fire role in support of infantry when attacking other infantry or fortified positions.
Historically the custom-built fully armored 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 gun turret limited these weapons' field of fire, but provided a simpler construction that was cheaper to build and less prone to mechanical breakdowns. The increased space and reduced weight of the turretless design also allowed mounting a larger weapon and providing heavier frontal armour on any given chassis, and in most cases these turretless vehicles also presented a lower profile as a target for the enemy.
Assault guns were primarily used during World War II, in the hands of the Germans and Soviets. 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 armored 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-barreled 152 mm howitzer mounted in an oversized turret. This was not a success in battle and was replaced with a very successful series of increasingly powerful 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). Armed with a high-velocity dual-purpose 75 mm gun, late production StuG III variants blurred the line between assault guns and tank destroyers. The Germans also built a number of other fully armored turretless assault guns, including the StuG IV, StuH 42, Brummbär and Sturmtiger. The latter two were very heavy vehicles, 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 contributed much to the German's ability to draw out the war long after they had lost the strategic initiative.
British and American 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. Among these was the M4(105), an M4 Sherman tank armed with a 105 mm howitzer. Also, the Mark IV version of the Centaur tank, the Mark V and the Mark VIII versions of the Churchill Tank were all fitted with 95 mm howitzers. As the amount of German armour encountered by the Allies decreased, especially in Italy, a number of American tank destroyer units were 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 forty pound (18 kg) HE-filled projectile (nicknamed the Flying Dustbin) 150 yards (137 metres). 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 would develop an improved air-droppable assault gun, the ASU-85, which served right through the 1980s. The US M56, and another similar 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 with 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. It is important, however, to remember that the Sheridan 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, with the US testing the M1128 Stryker MGS. The Centauro Wheeled Tank Destroyer of the Italian Army and the French AMX 10 RC heavy armored car are also good examples. 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 WWII.