The Evolution of Tank Armament: Rifled vs. Smoothbore Guns

On the 22nd of March 2021, the British government made an announcement that caught the attention of military enthusiasts and experts alike. They unveiled plans to upgrade 148 of their Challenger 2 main battle tanks, rebranding them as the Challenger 3. Among the myriad of improvements, one stood out prominently: the decision to replace the 120 millimeter rifled gun of the Challenger 2 with a 120 millimeter smoothbore gun.

To the uninitiated, this might seem like an odd choice. After all, for the past 200 years, the vast majority of firearms have utilized rifled barrels. The term “rifle” itself is derived from this very feature. Rifling involves cutting spiral grooves into the inside surface of a gun barrel. When a projectile is fired, it engages with these grooves, imparting a spin on it. This spin stabilization ensures the projectile is less likely to veer off course or tumble, resulting in increased accuracy. So, why would modern tanks opt for a smoothbore gun, a technology considered obsolete by the late 1800s?

To understand this, we need to delve into the history of tank armament. During the World Wars, tanks predominantly used rifled guns. However, by the 1960s, the landscape began to shift. In 1961, the Soviets introduced their T-62 tank, armed with a 115 millimeter smoothbore gun. This signaled a significant change in tank ammunition technology.

Historically, tanks utilized armor-piercing (AP) ammunition. These were essentially large bullets designed to rip through armor and cause significant internal damage. However, as armor technology advanced during and after World War II, the caliber of guns required to penetrate tanks like the Tiger II or IS-3 became impractically large. Two solutions emerged: APDS (Armor-Piercing Discarding Sabot) and HEAT (High Explosive Anti-Tank) shells.

APDS ammunition featured a narrow penetrator made of dense metal, usually tungsten. This penetrator was surrounded by a sabot, which sealed with the barrel and rifling. Upon firing, the sabot would discard, allowing the penetrator to achieve high speeds. HEAT shells, on the other hand, detonated upon impact, focusing the explosion into a narrow, hypersonic jet capable of piercing thick armor.

However, rifled guns posed challenges for both these ammunition types. For APDS, the required spin rate for stabilization became too high. For HEAT rounds, the spin diminished the effectiveness of the penetrating jet. The Soviets’ solution was the APFSDS (Armor Piercing Fin Stabilized Discarding Sabot). This ammunition was similar to APDS but had fins at the rear for stabilization. These rounds didn’t require spin, making smoothbore guns ideal for them.

Smoothbore guns also had another advantage: higher muzzle velocity. While rifling increased accuracy, it also reduced the projectile’s speed slightly. Given that APFSDS rounds already had good accuracy due to their fins, the added velocity from smoothbore guns made them formidable.

By the late 1970s, Western tanks like the Leopard 2 adopted smoothbore guns, with most NATO countries following suit. So, why did the UK lag behind in this transition? The answer lies in their preference for HESH (High Explosive Squash Head) rounds. These rounds, invented by the British during World War II, worked by squashing a large amount of explosive against a target and then detonating it. The resulting shockwave would produce spall, small chunks of metal that wreaked havoc inside vehicles. HESH rounds performed better when spinning, making rifled guns more suitable.

However, the introduction of composite armor and spall liners reduced the effectiveness of HESH rounds. While other nations focused on tank-on-tank warfare, the UK still envisioned its tanks supporting infantry. Rifled guns, with their ability to fire HE and HESH rounds, were better suited for this role. The recent switch to smoothbore guns in the Challenger 3 might be more about NATO standardization than purely performance.

In conclusion, while rifled guns have their merits, especially for certain types of ammunition, the modern battlefield dominated by tank-on-tank warfare has seen the rise of smoothbore guns. The Challenger 3, with its smoothbore armament, promises to be a formidable contender in this arena.