vulnerable to anything greater than small-arms fire. As the number of German tanks encountered by American forces steadily decreased throughout the war, most battalions were split up and assigned to infantry units as supporting arms, fighting as assault guns or being used essentially as tanks. In this sense they were an alternative to the Independent tank battalions that were attached to various Infantry Divisions. The expectation that German tanks would be engaged in mass formation was a failed assumption. In reality, German attacks effectively used combined arms on the ground, fighting cohesively. American tank destroyer battalions comprised three tank destroyer companies supported by nine security sections. The single-purpose tactics of the tank destroyer battalion failed to account for non-tank threats. In the 1950s the goal of providing airborne forces with a parachute-capable self-propelled anti-tank weapon led to the deployment of the M56 Scorpion and M50 Ontos. The concept later led to the M551 Sheridan light tank of the mid-1960s. United Kingdom British tanks in the early years of the war, both infantry tanks and cruiser tanks, were (with the exception of the pre-war Matilda I design) equipped with a gun capable of use against contemporary enemy tanks—the 40 mm Ordnance QF 2 pounder. This was replaced with the 57 mm Ordnance QF 6 pounder when that became available. There was extra impetus given to the development of anti-tank weaponry, which culminated in the 76mm Ordnance QF 17 pounder, widely considered one of the best anti-tank guns of the war. Towed anti-tank guns were the domain of the Royal Artillery and vehicles adapted to mount artillery, including anti-tank self-propelled guns such as the Deacon (6pdr on an armoured wheeled truck chassis) and Archer (17pdr on tracked chassis) and US-supplied vehicles, were their preserve rather than the Royal Armoured Corps. The self-propelled guns that were built in the "tank destroyer" mould came about through the desire to field the QF 17 pounder anti-tank gun and simultaneous lack of suitable standard tanks to carry it. As a result, they were of a somewhat extemporized nature. Mounting the gun on the Valentine tank chassis in a fixed superstructure gave the Archer, looking somewhat like the light-chassis German Marder III in appearance. The 17 pounder was also used to re-equip the US-supplied M10 Tank Destroyer, replacing the American 3-inch gun to produce the 17pdr SP Achilles. In 1942 the General Staff agreed on investigating self-propelled