The German manufacturer (Rheinmetall) that designed the 120mm tank gun used in most Western tanks reveled in mid-2016 that it had begun development of a 130mm tank gun a year earlier and was doing so without any customers lined up. There is a growing need for a larger tank gun, mainly because new armor protection designs (composite plus reactive armor) makes it increasingly possible for new tank designs to be protected against any existing tank gun shell, at least in the front of the vehicle. For years it was believed new anti-tank shell or missile designs would overcome this, but that is assured only by hitting the thinner side or top armor. There is still a big demand for the ability to succeed on the first shot with traditional attack at the frontal armor. Speed is an issue as well, so it’s up to a high velocity (tank gun) weapon.
The 120mm gun entered service in the late 1970s and for a long time there has not been a lot of demand for a larger gun. This new 130mm design may change that because the manufacturer revealed a 130mm details indicating it have similar weight (about 3.5 tons) as the 120mm gun and the same type (fixed, with projectile attached to the cartridge case containing the propellant) ammunition. Since the 130mm ammo will be larger and heavier than the current 120mm rounds which weigh about 22 kg (48 pounds) it will be more work for the gunner to handle and load a 130mm round that will weigh 10-20 percent more and be larger. Worse, a tank using the 130mm round will not be able to carry as many of them. None of these items are critical right now because Rheinmetall does not expect the 130mm L/51 (6.5 meter long) gun to enter service until 2025. That means Rheinmetall is currently seeking feedback from potential customers.
Since the 1980s there have been several efforts to design a larger (than 120/125mm) tank gun. Back in 2005 China was seen testing a 140mm gun on one its Type 98 tanks. Photos of test firings were released. Other countries have been investigating this sort of weapon, especially in the 1980s and 1990s. But no one has adopted the larger weapon for their tanks, mainly because the Russian super tank (equipped with a 152mm gun) that was supposed to justify it never appeared before or since the Cold War ended. Although the 140mm gun could fire an armor piercing round with twice the penetrating power of one fired from a 120mm gun (about 22 mega joules of energy, versus 11), the amount of ammunition carried was reduced by about a third (to 20-30 rounds, depending on the tank). The 140mm shell was about fifty percent larger than the 120mm one, and could probably knock out an M-1 tank with a frontal shot. The 140mm round weighs 36.3 kg (80 pounds), and is loaded in two parts (so a human loader can handle it). New Chinese tanks use an autoloader, so the round can be loaded in one piece.
Moreover, the recoil of a 140mm gun is pretty much the upper limit of what a tank can handle. The pictures appearing on Chinese television showed the 140mm gun mounted on one of the new, 50 ton, Type 98 tanks. But China is known to have a heavier (over 60 tons) Type 99 design (bought from Russia, which never adopted this particular model). The 140mm gun (which appears to be of German design, technology which was either bought or stolen), would be a stretch on the 50 ton Type 98, causing structural and maintenance problems, and perhaps even restrictions on how often the gun could be fired. The recoil force of a high-speed 140mm round creates some serious engineering problems for any tank mounting it. China decided not to mass produce too many Type 99s, even without the 140mm gun. That version would be too heavy (at about 60 tons) for many Chinese bridges, not to mention railroad equipment. Most Chinese tanks are closer to 40 tons. Instead China settled for a 54 ton Type 99 armed with a 125mm gun. A tank with a 140mm gun would be useful against the most modern tanks now used by Japan, South Korea, and the United States. But it would be cheaper to buy modern anti-tank missiles if you wanted to go after these high-tech tanks. However, China may fear that these countries will equip their tanks with one of the several anti-missile systems appearing on the world market. China never followed up on their 140mm gun design but the new German project appears to be addressing some of the issues (size, recoil, loading) encountered with earlier 140mm and 152mm designs.