American AT Guns
When the US army entered the war their standard anti-tank gun was the M3 37mm, this was a light and effective gun plus 37mm was considered to be the standard calibre used in the interwar period. Each US infantry regiment had a motorised anti-tank company containing twelve such guns plus 169 personnel. By 1942 this gun was found wanting when pitted against German heavy tanks so by 1943 the M3 was replaced by the heavier 57mm M1, the M3 still had a role to play and many were used in the pacific theatre not only against the lighter Japanese tanks but as an infantry gun, the M3 fired a small but effective canister round which was devastating against infantry attacks.
The 57mm M1 was issued along the same lines as the M3 with a regimental anti-tank company of twelve towed guns. The M1 was based on the British 6 pdr and was widely used until the end of the war.
As early as 1941 the US army requested a heavier gun to combat the increasing threat of German heavy tanks. After a few designs they finally tried a weapon based on both the 3in anti-aircraft gun and the M2 105mm howitzer, the result was the M5 3in anti-tank gun and it first made an appearance in 1943, early results were good but the gun was too heavy to be of any practical use, which resulted in it being issued to tank destroyer battalions rather than infantry regiments.