In modern military science, big guns such as cannons, howitzers, or mortars operated by crews and of a calibre greater than 15 mm. The earliest artillery, introduced in the 14th century, were cannons and mortars of bronze, brass, or iron mounted on two-wheeled carriages. Modern artillery dates from the second half of the 19th century, when advances included steel gun barrels, more powerful gunpowders, and piston mountings that held artillery carriages steady during recoil. Both powder and projectile were encased in a shell, which allowed for faster loading. Since World War II, artillery has been ranked as light (up to 105 mm, for support of ground troops), medium (106–155 mm, for bombardment), and heavy (over 155 mm, for attacking rear installations). See alsoantiaircraft gun.