Why would a tank have multiple main weapons or a main weapon with multiple barrels?

Question

Fiction is filled with examples of tanks with multiple main weapons, from Command and Conquer's Mammoth to Warhammer 40,000's Baneblade to World of Tanks's IS-2-II (although the IS-2-II was based on a set of real-life designs, it ultimately only took to the battlefield in a video game).

Ultimately, the closest we got to this in real life was the Nazi Germany-built Maus superheavy tank, whose primary armament was a 128-millimeter anti-tank gun and whose secondary armament was a 75-millimeter infantry support cannon.

But why? Other than for the sake of the fact that they look cool, why would somebody design a tank with either multiple main weapons a main weapon with multiple barrels? Are there tactical or technological reasons for why such a thing would be designed?

Answer 1

It might no longer be a tank as we know it. Or perhaps it would ...

Look at the M2 Bradley IFV. It is armed with TOW missiles against tanks, an autocannon against light armored vehicles, a coaxial MG against troops, and originally firing ports for small arms against troops. Critics complained that it was a jack of all trades and master of none.

By comparison, the M1 Abrams MBT has a main gun against enemy tanks and other distant, relatively large targets, a coaxial MG against troops, and a pintle-mounted MG against troops and some air targets. (Modernization added/changed the secondaries.) The Merkava has more MGs and a smallish mortar.

For the IFV, the different target characteristics caused two different "main guns" -- the TOW against a few very hard targets, the autocannon against medium-hard, more numerous targets. The MBT added different main gun ammunition instead of a different main gun. Historically, tanks were anti-infantry and anti-artillery weapons, not anti-tank weapons, while anti-tank was left to other systems (tank destroyers, towed anti-tank guns ...). They all retain this role to various degrees.

So you would need a situation where the two roles -- fighting tanks and fighting less hard targets -- are better served by different weapons systems on the tank, not by different ammunition for one weapon or by different, complementary vehicle families.

• Railguns become viable. They are optimized for firing hypervelocity, low-caliber, high-density penetrators (like APFSDS without the sabot). In addition to those, the tanks mount something howitzer-like for explosive shells, smoke, WP, etc.

• Lasers become viable. They are suitable for relatively soft targets like troops in the open or trucks, and for that job they are better than explosive shells from a traditional tank gun. So they are mounted in addition to the main gun.

Answer 2

Its been done before.

The Ontos was an interesting beast - but essentially its useful when you want a high volume of fire, but can skimp on either the size of the round or the ability to reload. Essentially, you're unloading large volumes of fire at once (and 105mm is roughly the size of a light tank gun) - and the ability to load beehive rounds is a bonus. In this case you had the potential for a knock out punch of 6 rounds, or 6 shots before you needed to reload.

Another alternative to look at is SPAAG - like the famous ZSU - lots of smaller autocannon, designed for high volumes of fire.

Fundamentally you're not designing tanks to fight tanks the same class. You're either building a glass cannon that has one chance to take out an enemy tank with overwheming firepower, or something designed to kill 'softer' targets.

Answer 3

The number of weapons mounted on a fighting vehicle depends on multiple, conflicting needs.

• You want to be able to kill the enemy.
  Your weapon must be capable of defeating the enemy's armor, or capable of spewing enough bullets to defeat their combined evasion + ability to absorb damage.

• You want your vehicle to be as light, fast and maneuverable as possible.
  Mobility is defense.
  It's no good having a vehicle that can destroy anything, and the enemy can just stroll away from it. For example, the A-10 Warthog airplane has enormous firepower, but makes for a very poor air-to-air interceptor.

• You don't want to apply humongous overkill.
  It's no good being able to 100% certain kill that infantryman, if you need to use a $5million super selfguiding missile for each one. Aside from busting your bank, you will be unable to carry enough ammunition along.

So, each fighting vehicle's weaponry load must match the technical and financial means of its owner, and must also be matched to its operational role and target.

So finally, to answer the question: Why would a tank, specifically, have multiple primary weapons, or a multi-barrel primary weapon?

If the tank's enemy is weak enough to remove the need to mount the ultimate best cannon portable, and the enemy is numerous or evasive enough to require multiple shots to defeat, then multiple similar barrels would be appropriate.

In real life however, the mass and cost and engineering difficulty of mounting two or more of the same weapon is usually more than the preferred alternative: upgrading the firing speed of the single weapon.

Again with the A-10 Warthog as example: Does it have one primary cannon, or 7?

And lastly, just what is considered to be the primary weapon?
Consider the Abrams tank:
Its "primary weapon" is the 120mm smoothbore cannon. Yet this weapon caries only 40 rounds of ammo, and is rarely used.
Its 50cal machinegun carries a lot of ammo (900 rounds), and is used a lot more often. And it also has two .308 machineguns, that are supplied with a staggering 10400 rounds of ammunition!!!

Answer 4

Based on the examples you give, I'm assuming you mean a tank that mounts multiple cannon caliber (generally 20mm or greater) guns, which is quite rare, not multiple guns of any size, which is almost universal, or multiple weapon systems (cannon + missiles for example) which is less common but not unusual.

Single Shot Weapons

One possible reason is because it uses something like a "recoilless" gun to increase the firepower it can mount, at the cost of making each gun a single shot weapon. The vehicle below is the M-50 Ontos, which weighed about 9.5 tons (when a contemporary MBT weighed about 50 tons) that nevertheless mounted 6 105mm recoilless cannons (same caliber as a contemporary MBT). Each cannon could only be reloaded from the outside of the vehicle, hence the multiple barrels.

Manufacturing Limitations

Another reason a tank might mount multiple main guns is that you want to mount a larger or more powerful gun than you can fit in a turret, but you still want a turreted gun. In the case of the M3 Grant/Lee as seen below, US manufacturing at the start of WWII was not capable of building a turret large enough for the 75mm gun that was needed to deal with contemporary tanks. Initially, the plan was just to mount the gun in a centrally located casemate like a StuG III/IV. However infantry commanders demanded a turreted gun, so the design was revised to mount a 37mm cannon in a turret, and the 75mm gun was moved to a side sponson. The limitations of this design were obvious even before it saw action, but it was deployed anyway as a stopgap while the US built up its arms industry.

Heat Dissipation

Another possibility is that electromagnetically launched weapons like railguns or coilguns become viable for a vehicle as small as a tank. Almost all the waste heat generated by a railgun or coilgun winds up heating the weapon itself. In a conventional gun a lot of that heat is carried away by the exhaust gas or the casing of the projectile, so the limiting factor on rate of fire tends to be the loader/autoloader. For railguns/coilguns loading could be at least as fast as for a cannon, and will likely be much faster (you only need to load the projectile, without propellant), but the armatures/coils will be experiencing greater heating than a gun barrel and may even be less heat tolerant. As such, heat dissipation will probably be the limiting factor in rate of fire. If heat build up is so significant that it impairs performance a second barrel might be a worthwhile improvement, as long as the guns themselves are not too large.

Answer 5

Opener and followup

This is probably dumb, but what if you develop a system pairing an armor-penetrating tungsten or depleted uranium shell fired from one barrel and a high explosive shell fired from the other? You would want them to fire at almost the same time, offset by some small fraction of a second, so that the HE shell hits the same point as the AP shell, where the armor is most compromised.

You could imagine needing to do something like this if your adversary has developed extremely well-armored tanks, such that the weight of a combined sabot+HE shell capable of penetrating the armor becomes impractical. Or maybe in addition to thick armor, they have an active defense system capable of destroying the HE shells, but wouldn't be able to affect the trajectory of the AP shells. If the active defenses trigger harmlessly on the AP shell then take some time to re-arm, the HE shell could then sneak past.

Answer 6

As many answers have shown, having multiple primary weapons on a tank is normally not done. There are lots of examples of secondary weapons and one multi-barrel primary weapon but that vehicle shouldn't be called a tank. You'll have to leave the realm of modern tanks to come up with a good use case. I can see two concepts that lead to multi-barrel primary weapons:

1. Your primary weapon isn't really the barrel in the first place. You have a prime source that can cycle a lot faster than what does the shooting--probably a matter of cooling. You have a fairly simple barrel that absorbs so much energy in firing that you have to cool it for a while before you can use it again. Perhaps you're hitting a lithium deutride pellet with enough laser power and have some way of directing the energy downrange. The lasers are down in the body of the tank and can be directed to one of a group of barrels.

2. Your tank is big enough it can mount multiple weapons of the largest feasible size. You'll find a good example of this in history, albeit not of a tank: battleships. BOLOs aren't exactly practical without some technological breakthrough that favors the defense. Battleships had many guns because there was little reason to make bigger guns and the square-cube law made big platforms a good idea.

Answer 7

Ultimately one designs tanks not to reach the mall down the road and buy the latest one foot with ham, but with the intent of achieving supremacy on the battlefield and win the battles and the war in which those tanks are used.

Sadly, enemies have this nasty tendency to research countermeasures to the weapon you use to attack them: they hide in trenches when you shot at them with bullets, they use thick concrete walls when you cannon them, they go in underground bunkers when you nuke them. What's a well meant warlord supposed to do to win a battle?

Of course equipping multiple types of weapons on a single attack vehicle will increase the possibility for it to win the engagement thanks to its versatility, and will encumber the enemy with having to use multiple countermeasures at the same time.

Answer 8

Depends on your definition of "Main Weapon". I'm going to assume you mean weapons of larger than 12.7mm/.50 calibre.

The short answer would be flexibility. The modern 120mm gun is very good at destroying other tanks. When loaded with High Explosive (HE) rounds it is also quite good against hardened static targets like bunkers.

However, it is quite useless against aircraft for example. A cannon in 20 or 30mm with a higher rate of fire is suitable for this role. It would also be good for destroying light vehicles that do not require the main gun. The AMX 32 had such an arrangement.

The Israeli Merkava mounts a 60mm mortar to supply close infantry indirect fire support.

In the future, if the Main Battle Tank's primary armament became a rail gun, then a coaxial conventional gun in 50 to 75mm with a decent rate of fire and multipurpose HE projectiles could fulfil a wide range of battle field roles that the main gun's high velocity projectile could not.

The main reasons these things are not commonly done today are space and weight. If something happened that allowed tanks to be larger but no heavier reducing the penalty of a second weapon system and its ammunition (much lighter armour for instance) then designers would probably fit them.

Answer 9

A difference in Doctrine

A tank with multiple primary weapons would be equipped to target multiple enemies and destroy them simultaneously. Essentially a brawler to the modern MBT's Spear or the IFV's short-sword.

The Brawler-Tank fits a doctrine which doesn't care about losses and wants to bring maximum firepower in as small a package as possible. Alternately, it fits a doctrine that entails fighting wildly varied enemies without requiring the tank to be supported by more specialised vehicles/infantry.

Both of these are portrayed in the Warhammer 40,000 franchise by the Imperial Guard, which field a wide range of tanks, most of which mount a variety of turrets, sponsons and hull-mounted weaponry.

The principle tank of the Imperial guard is a Leman Russ Battle Tank, which has a large-calibre turret for anti-armour work, a forward-mounted machine-gun (technically more of an automatic grenade-launcher, but lets not get bogged down) and two manned sponsons on the sides which mount anything from flamethrowers all the way up to anti-tank lasers.

The result is a vehicle which can face virtually anything and have a weapon to deal with it.

Which is important for an army which routinely fights armies with tech-capabilities ranging from scrap-metal cars out to haunted living-metal hoverships, passing through swarms of space-locusts and flying powered-armour..

Even if the tank is outclassed or its main cannon is useless against a given target, one of its secondary weapons is probably going to do something, so a tank-battalion should bring enough of those secondaries to get the job done.
That might result in heavy losses, but the imperial guard always has reserves...