The difference between tanks and self-propelled guns on short notice is in their role and everything needed to perform that role. Although models, these two vehicles in the picture above show why the self-propelled gun (right) is often confused with the tank (left) by the average person.
A modern main battle tank (MBT) is a direct fire weapon with a primary anti-AFV (Armoured Fighting Vehicle, e.g., other tanks, infantry fighting vehicles, self-propelled guns) capability and a secondary soft target capability. At the same time, a self-propelled gun (SPG) is usually an indirect fire weapon against soft targets with a secondary role against fortifications and armor. Direct fire is more accurate and kinetically robust than an indirect fire of the same caliber but has a much shorter range and can only be used within line of sight to the target.
To this end, the self-propelled gun is usually protected against infantry small arms, heavy machine guns, and artillery fragments in case it is surprised by reconnaissance units or caught in counter-battery fire. It is not protected against anti-tank weapons since it is meant to be behind the lines. Its gun is a howitzer, often based on a towed model, and is capable of well over 45 degrees elevation. However, it can be lowered to horizontal and used as a direct fire weapon against enemies approaching the gun’s position; it doesn’t have the sophisticated optics, including night vision and laser range-finding, that a top-of-the-line tank does.
A lot of SPGs bear a passing resemblance to tanks since they often have turrets and tracks, and many use tank chassis as the basis of their design, but some are wheeled, and there have been several since World War 2 that are open-topped or even unarmoured.
Self-propelled gun turrets are broader and higher than tank turrets – they don’t need to present a tiny target with a low silhouette, but they do need room to store larger ammunition and for a gun crew or autoloader to operate.
The main battle tank is armored across the frontal arc against anti-tank threats it can expect to encounter and armed with a high-velocity cannon firing specialized armor piercing solid shots. It can fire other shells, too, such as high explosive (HE), but these fire at a lower velocity than the armor-piercing penetrators. A 120mm HE tank shell is generally similar to the same sized HE artillery shell. Still, due to the shallow trajectory, terminal effects are spread over a more limited area and said shells usually have less explosive than a 120mm HE mortar bomb.
However, the thicker, stricter walls of the shells offer improved fragmentation than the thin steel casings of mortar bombs. This means a smaller blast radius for tank and artillery shells compared to mortars, but paradoxically a larger kill zone as more – and larger – fragments are created, traveling further than lighter and smaller ones from a mortar. The exception is HEAT (High Explosive Anti-Tank) ammunition, which has a relatively reduced fragmentation effect and relies on a focussed blast. Although used in place of HE as a dual-purpose munition in tanks, it is a comparatively poor anti-personnel weapon.
So, for a given caliber of weapon, the blast effect of conventional HE is more significant for individual mortars, tank guns, and artillery. The reverse is true for their relative lethality (lethality is measured as a radius within which 50% of personnel will be killed). Even so, a single 120mm mortar round killed 68 people, and 144 more were wounded in an attack on an unsuspecting crowded marketplace in Sarajevo in 1994.
However, indirect fire is usually a weapon of mass effect. It works by blanketing a target with effective interlocking radii, smashing targets with blasts from different angles and suppressing the enemy’s ability to act effectively, or even neutralizing heavy weapons and vehicles (suppression means forcing personnel to take cover and psychologically assaulting them, neutralization mean killing personnel, and destroying optics and other delicate components). Therefore lower per-round lethality is offset by the number of rounds detonating in the target area. The reduced lethal radius means infantry can advance closer to a barrage, ready to move onto the target as soon as it is lifted.
Direct fire, in contrast, relies on having a line of sight to the target. Accuracy is essential to pick off targets with individual shells rather than saturate an area.
Of course, there are exceptions to every rule, and recently guided artillery munitions allow exact targeting of a tank or fortification by a specialized anti-armor or bunker-busting shell, negating the need for saturation fire of such hardened targets and fulfilling the role of direct fire support in some ways.
In World War 2, self-propelled artillery was often used as direct fire ‘assault guns,’, especially by the Germans and Soviets. The assault gun gave attackers a mobile cannon that could direct its fire at enemy points of resistance as commanders encountered them on the front lines. It was based on a tank chassis with a fixed, armored superstructure instead of a turret. Since it was intended to support front-line troops by knocking out strong points where anti-tank defenses might be expected, it was typically well-armored on the front.
The Germans, after 1941 found their assault guns would often encounter tanks or be called upon to attack them as impromptu tank destroyers, so from 1942 began fitting their 75mm howitzer-armed SturmGeschuetz (StuG) IIIs with a dual purpose 75 mm anti-tank guns instead, supplementing the initially rare long-75mm armed Panzer IV tanks. Without a turret’s extra weight, the chassis could carry a bigger gun than the tank version. The StuGs proved themselves effective tank destroyers during the more defensive campaigns from 1943 onwards, their low silhouettes suiting them to ambush positions in the close country of the Normandy bocage, for example. Despite this, they remained under the control of the artillery and were crewed by gunners, not tankers.
The Soviets first used open-topped SPGs with 76.2mm gun-howitzers, fulfilling the dual roles of mobile anti-tank and infantry support weapons. However, they soon began mounting increasingly heavy artillery (122mm and 152mm) in casemates, similar to the design of the StuGs. However, these guns were howitzers used in the direct fire role for infantry support against buildings and bunkers, not high-velocity anti-tank cannon, and relied on the sheer explosive power of the large shells to disable enemy tanks. They were so effective in this role, even against German heavy armor in the last years of the war, that they were nicknamed the ‘beast killers’ or ‘big cat killers.’
Today self-propelled guns have access not only to smart munitions but also to an array of carrier rounds that dispense sub-munitions, sensors, smoke, leaflets, and many other payloads, as well as having the use of fuzes that offer point detonation, airburst, and delay options. This increases their utility over the tank gun, which is specialized for direct fire only (although the latest developments by arms manufacturers like Rheinmetall introduce electronically timed fuzes to facilitate airburst and behind-cover effects for tank shells).
Self-propelled guns may look like tanks to the untrained eye, but they are very different beasts.
* The article was written on the basis of the original article from Dom Hyde, a former artillery Royal Artillery.