Updated 6/9/2017 to correct an error concerning the Rheinmetall 120 mm’s users.

German arms manufacturer Rheinmetall recently unveiled its new highly-anticipated 130 mm cannon design for use on next-generation tanks. Rheinmetall’s 120 mm tank gun is the most widely fielded non-Russian tank gun and holds a virtual monopoly on NATO and Western-allied tank armaments; currently, most Western main battle tanks (MBTs) are armed with smoothbore 120 mm Rheinmetall cannons, except the UK which uses a rifled 120 mm cannon, and the French who engineered their own CN120 120 mm smoothbore. The Rheinmetall gun has two variants, the L/44 and L/55, which have barrels of different lengths. In many cases, the cannons are license-produced, such as the L/44 M256A1 cannons used on M1A1 and M1A2 Abrams tanks, which are manufactured at the Watervliet Arsenal in New York and have some alterations including a coil-based recoil mechanism instead of a hydraulic one. All of these 120 mm cannons have generally similar ballistic performance and will be referred to collectively.

Intended for use in next-generation main battle tanks, the new gun 130 mm comes at a time when tank armor is becoming increasingly difficult to penetrate. Innovations such as explosive reactive armor, high-end composites, and active protection systems oftentimes mitigate if not completely nullify the effects of anti-tank weapons such as kinetic (KE) sabot rounds and explosively formed penetrator warheads. Russia, in particular, fields Kontact-series explosive reactive armor, which is designed to, among other things, reduce the effectiveness of the of KE rounds fired from 120 mm tank cannons. Equipped with Kontact ERA, many newer Russian tanks have armor levels which approach that of advanced NATO tanks and inhibit the penetration of all but the highest-power anti-tank weapons.

It is in this context that the 130 mm gun becomes appealing. Because increases in bore diameter are not proportional to increases in propellant volume, the modest 10 mm increase in bore size actually yields a massive 50% increase in kinetic energy over the standard 120 mm round. This increase ensures that the 130 mm gun will remain potent for some time, whereas the ability of the 120 mm gun to counter future advances in protection is less clear.

However, it remains unseen whether or not the 130 mm gun will eventually achieve the ubiquity of its predecessor. Upping the caliber of tank cannons to increase kinetic energy is far from a novel idea. In fact, trials during the Cold War were undertaken with even larger 140 mm guns. However, a number of problems precluded the introduction of the 140 mm gun, and it is easy to see how these same issues may manifest themselves in tanks with the 130 mm as well.

The biggest issue that will face engineers trying to incorporate the 130 mm gun is its increased weight and dimensions. The 130 mm is heavier, at 3.5 tons, than the 3 ton 120 mm gun. Housing the slightly enlarged 130 mm, as well as its correspondingly enlarged equipment, will necessitate a redesigned turret, according to Rheinmetall engineers. This could be problematic because many NATO tanks are already behemoths. The American M1A2 SEP V2 Abrams weighs in at a staggering 69 tons, a weight similar to the largest heavy tank destroyers of World War II.

At this point, it is worthwhile to examine the role of the MBT on the battlefield. Unlike the highly specialized 70-ton gun carriages of World War II, main battle tanks are not slow breaching vehicles. Rather, they are all-purpose armor intended to fill a variety of roles, from engaging other tanks to exploiting maneuver opportunities and bolstering infantry operations. Thus, the “main” in “main battle tank” reflects that MBTs are intended to be versatile, which requires speed and mobility. Further increases in weight could threaten this mobility significantly.

Most obviously, there is the issue of operational mobility, or the ability of the tank to move to the battlefield under its own power. One factor affecting operational mobility is ground pressure per unit area, usually expressed in psi. This figure is obtained by dividing the tank’s weight by the surface area of its tracks’ contact patch. Obviously, a higher weight per unit area results in the tank being more prone to sinkage into soft ground as well as more rapid wearing of road surfaces. If the weight of a 130 mm-armed tank were to be increased but the track size was to remain the same, problems associated with higher ground pressure would manifest.

Alternatively, when working with added weight of the 130mm, tank engineers could work to increase the track surface area proportionally to the weight increase and thus achieve a low ground pressure. However, tanks with tracks of a larger length and width have their own issues. The larger the vehicle, the more difficult it is to fit on narrow roads, through urban streets, and over bridges. No matter the track size or ground pressure, overall weight also plays a factor: excessively heavy tanks may be unable to pass over weak bridges without risking collapse, which is problematic when there are many rivers in the area of operations.

Also important is the effect of weight on strategic mobility, which is the ability to move the tank from where it is based to where it is needed for combat. Tanks generally do not move unassisted over long distances, because they have atrocious fuel economy and the wear incurred by such travel is expensive. And, of course, if there is a large body of water such as a sea or ocean, boats are a necessity. Strategic airlifters such as the C-5 can move tanks very rapidly, but this is by far the most expensive option and is only available to countries that possess such aircraft. Tanks are usually moved over land by a prime mover, whether it be a train or a truck, for the aforementioned reasons. Trains and trucks have payload size limits, namely the width of roads and the maximum width of a train car as permitted by rail infrastructure. Airlifters have similar limits to their maximum cargo dimensions and weight. Thus, the limits of tank size are not only established by battlefield needs but also logistical ones; for economic as well as tactical reasons, all tanks produced must fit onto some sort of strategic transport.

So, in conclusion, tank designers will have to avoid significant increases in weight and size when producing new tanks with the 130 mm cannon. Increases of more than a few tons over current-gen tanks would likely be unacceptable, and such increases could be wrought on not only by the inclusion of a new gun but also of new protective systems, drive technologies, etc. Because the 130 mm cannon will be heavier, other components will need to get lighter.

Given past trends in tank warfare as well as the trajectory of aircraft and ship weights, this seems a tall task — tanks, planes, and indeed most vehicles have tended to become larger and heavier as time passes. Whether tank designers can halt this trend while still incorporating the larger 130 mm gun remains to be seen.

The issue of increased cartridge weight is another important aspect. The cartridge, which contains the shell and its propellant, is loaded into the breech of the gun the gun before it can fire. The 130 mm cartridges are 30 kg, whereas the 120 mm cartridges are 21 kg. Because of this significant 9 kg increase in weight, Rheinmetall engineers believe that the 130 mm cartridges must be autoloaded. Autoloading is the process by which mechanical means are used to reload the gun, as opposed to manual loading, in which a human loader performs the task.

On paper, autoloading seems attractive. After all, machines tend to perform simple tasks (such as loading a gun) faster and more precisely than humans do. However, there are actually many benefits conferred by using a human loader. Counterintuitively, a well-trained human loader can potentially load rounds more quickly than current autoloaders, under ideal conditions. And, while machine loading may sound precise and reliable, autoloaders are highly complex and thus can break down or fail, which can be catastrophic in combat. Many tank commanders also like having the extra crewman for a number of reasons, including the increased manpower for odd tasks and the redundancy in case of casualties. Because of the numerous benefits of manual loading, it is the method currently preferred by the majority of NATO countries, including military heavyweights such as the US and Germany.

Autoloaders do have some advantages, the largest being their smaller size relative to a human loader (or, more precisely, the space required to house a human loader in the turret). The usage of an autoloader is one of the reasons that Soviet tanks such as the T-72 and T-90 have a lower profile than manually loaded Western tanks, thus presenting a smaller target. This decrease in turret volume afforded by autoloaders could play a role in compensating for the increased size and weight of the 130 mm gun, assuming the necessary autoloader is still smaller and lighter than a human loader and its facilities. Another advantage afforded by the autoloader is its indifference to fatigue and bumpy terrain (human loaders can be jostled around if the tank is careening through ditches or over bumps). It is also possible that engineers could produce an autoloader that is faster and more reliable than a human loader in the future.

In conclusion, there are barriers to introducing the 130 mm tank cannon. Not only will it be difficult to stop weight and size increases from impacting mobility, but it will also require some NATO members to adopt autoloaders, which they have opposed for decades. However, the 50% increase in kinetic energy is quite a dividend and could make all the engineering effort well worth it. If Russian tank armor and defenses progress much further, adoption of the 130 mm may be the only way forward for Western tank armaments. In any case, it will be around a decade before the 130 mm gun is ready for mass production and probably even longer before a tank is designed around it, so the best that can be done is to wait and see, as the progression of armor and resulting changes in NATO penetration requirements will not be revealed for some time.