The search for a more suitable, longer-range delivery system took up much of the immediate post-war era. The US invested in the recoilless rifle, delivering a widely used 75 mm design, and less common 90 mm and 106 mm designs (the latter was usually mounted rather than infantry-handled). The 106 mm formed the basis of a dedicated anti-tank vehicle, the Ontos tank, which mounted six 106 mm rifles. The Australian Army also fitted M40 recoilless rifles to Land Rover Series 2 vehicles for use in an anti-tank role. The Soviet Union also built recoilless rifles in various calibers intended to be used as anti-tank weapons, most commonly 73 mm, 82 mm, and 110 mm (only the 73 mm remains in service with the Russian military today, though the other two can be found all over the world due to Soviet military aid during the Cold War). The British used a 120 mm (4.7 inch) design to equip infantry units, the BAT series, which served from the 1950s until replaced by MILAN, but it was generally too heavy for infantry use and had to be towed by, or mounted on, a vehicle for maneuverability.