Rifle Bore Vs. Smoothbore – The Tank Gun, Which One is Better

 

A Main Battle Tank or MBT or just a tank is the most destructive and powerful land assault system for an army column. The tanks weigh anywhere between 40 tons (medium) to 68 tons (heavy) to 25 tons (light) and are key to any assault that it spearheads. Tanks are mobile, armored, heavy weapons platforms and the tank gun is the most critical part of the tank. Most of today's modern-day tank guns are above 100mm (105 mm, 115 mm, 120 mm, 125 mm) with 120mm being the most common.

The tank guns are housed in a rotating turret which allows the tank to engage enemy targets at just about any orientation. There are 2 types of tank guns that are differentiated by the make of barrel, smoothbore and rifle bore. A smoothbore barrel is completely smooth on the inside, cut into the metal; while a rifled-bore barrel has spiraling grooves cut on the inside of the barrel, that help guide a tank shell down the length of the barrel.

Tank guns today are compulsorily equipped with a bore evacuator that helps prevent toxic gasses emanating from a fired shell from leaking back into the crew compartment after the tank crew has fired the shot, and releasing it into the low-pressure zone. This allows the tank crew to open the tank gun breech to load the next shot without being forced to inhale any harmful gas. The effective range of a tank gun depends on ammunition used, tank man training, targeting sights used, whether a tank is stable or mobile, weather conditions, and others.

In a rifle bore tank barrel, as the projectile moves down the barrel, the grooves grab the sides of the projectile, imparting a spin that stabilizes the projectile in flight and trajectory making them extremely accurate over long distances. Smoothbore tank guns have a smooth barrel but these are less accurate and used for short-range firing but can fire tank shells with much greater force due to less friction.

Historically cannons had smoothbore barrels but as time progressed rifle bores gained prominence. Rifle bore tank barrels are best for shooting conventional tank shells, that use rotation to maintain stability over greater distances than a smoothbore. After World War 2 APDS (Armour Piercing Discarding Sabot) and HEAT (High Explosive Anti Tank) gained prominence. Fired from rifle-bore tank barrels, they were effective but as tank armor thickness increased their effectiveness waned.

So, tank barrels went back to smoothbore and the 1^st tank seen was Soviet T 62 in the 1960s with 115 mm 2A20 Molot gun and on NATO tanks from 1990 onwards. A smoothbore tank barrel’s reduced resistance meant a tank shell of whatever caliber could now travel at much higher speeds, to a greater distance, and without spinning, ideal for HEAT rounds. But APDS must spin, so APFSDS (Armour Piercing Fin Stabilized Discarding Sabot) were developed, though spinning allow tank shells to travel at much higher speeds, to a greater distance.

As such most modern Main Battle Tanks (MBTs) today use a smoothbore barrel, with a few exceptions like the British Challenger 2 tank and the Indian Arjun tank that have rifled bore barrels. Both these tanks use HESH (High Explosive Squash Heads) shells that contain a plastic explosive in them. Arjun which uses a 125 mm rifle bore tank gun also fires thermobaric rounds. The explosive flattens against armor before detonating, resulting in large chunks of metal peeling off on the inside, working like shrapnel piercing some of the heaviest armor.

A rotating rounds’ centrifugal force aids in the flattening of the explosive. Though rifled bore barrels allow the firing of more variety of shells, smoothbore tank barrels are much cheaper. The coefficient of friction in a smoothbore is less than that of the rifle bore tank barrel. As a force of friction is equal to the coefficient of friction times the normal force; it means that a larger propellant charge can be used on the shot with less risk of causing any kind of damage to the barrel or shot. As such it may look like smoothbore scores over rifled bore easily.

The German Rheinmetall 120 mm smoothbore gun is the most used smoothbore tank gun, used in multiple countries worldwide on Leopard 2 tanks as well as Abrams tanks. However, note that everywhere in the world AFV or Armoured Fighting Vehicles other than tanks like Bradleys, BMPs, and others; fire Armor Piercing shots such as AP, APIT, HVAP, and APDS from a rifled bore. Though it may happen that when a tank shell while moving out of a rifle bore tank barrel, if not fully sealed against the rifled grooves; chances are high that the propelling gas may escape and exit the gun barrel first before the projectile exits. Similarly, the Russian 125 mm 2A46 (D-81) smoothbore gun is the most widely used tank gun in Russian/Soviet design or copied tanks and its derivatives.

So instead of increasing the tank shell’s velocity, the gas pressure has to propel the shell to move forward along the rifled bore while also increasing its velocity, resulting in loss of energy. This is more or less the reason why rifle-bore tank guns perform unsatisfactorily at longer ranges compared to smoothbore Armor Piercing Fin Stabilised Discarding Sabot. Another point that goes in favor of smoothbore tank guns like Rheinmetall 120 mm, is that the gun comes in two versions; shorter barrel length L44, and longer barrel length L55. Both fire the same ammunition, but L55 helps generate a higher velocity,

Furthermore, continuous friction causes excess wear and tear on the rifled grooves of the bore that requires frequent cleaning; resulting in a faster reduction in the life of rifle bore tank barrel requiring quicker replacement than a smoothbore barrel, where the wear and tear is much less pronounced. As such even though accuracy is an issue smoothbore tank barrels are best suited to hit hard and take out hardened targets.

Whatever the drawbacks in smoothbore tank barrels, these can be compensated by using modern ballistic computers and optics for faster target acquisition, target engagement, and follow-up shots. But if penetration isn’t the target then a rifle bore tank gun is best that can fire everything from HESH, HEAT, HE, PCB, and thermobaric rounds that achieve their objective by way of sheer kinetic energy force. As such for all purposes, the smoothbore tank gun seems to be scoring over rifle bore tank gun, but the latter has its own advantages. So, the debate about which tank gun is better, is likely to continue.