were up-armored M4 Sherman "Jumbos" which were used as assault guns. Both the US and UK developed very well-armoured and armed tanks intended for assaulting heavily defended areas - the T28 GMC and 'Tortoise' had casement designs and weighed around 80 tonnes but did not enter service. Cold War The immediate post-war period saw the final fielding of heavy tanks, including the US M103 heavy tank, the British FV214 Conqueror, and the French ARL 44 (in very limited numbers for the ARL 44), all in response to the Soviet heavy tanks of the period. The largest tank guns were approaching maximum calibre whose shell could still be handled by the crew, even using awkward two-part ammunition (separate projectile and propellant case, similar to battleship guns), which greatly slowed their rate of fire. Thanks to improved shell designs and fire control technology improving accuracy, postwar medium tanks were catching up to heavy tanks in firepower. The tactical value of heavy tanks thus declined to the point that no new designs were fielded; the heavily armed mediums came to be known as the main battle tank (MBT). Doctrine held that less expensive self-propelled artillery could serve in the infantry support role. The weight of MBTs quickly increased during the Cold War, and most third generation MBTs including the M1 Abrams, Challenger 2, Leopard 2, Merkava, Arjun MBT, and Type 99 have weights similar to those of 1950s heavy tanks. Older heavy tanks with steel armour were rendered obsolete by anti-tank guided missiles and high-explosive anti-tank (HEAT) ammunition. The much more flexible missiles are effective at ranges beyond a tank gun's range, and sheer armour mass was no longer a guarantee of survivability against the largest HEAT warheads of tank guns or missiles. See also Light tank Medium tank Super-heavy tank Main battle tank Notes References Tanks by type