6lb Gun

The QF 6-pounder was a 57mm anti-tank gun developed by the Brits to replace the outdated Qf 2-pounder AT gun. It is notorious for being the weapon that could be found on many armored vehicles throughout the war, and was so successful that it was adopted by the US Army to replace their own outdated 37mm guns in Europe.

IRL

As far back as 1938, the QF-2 pounder was rapidly being seen as outdated, particularly given the advances in tank design that were occuring at that time. The 57mm was settled upon due to the fact that the navy used to use that caliber of weapon, and thus it would be easier to manufacture more than anything else. The gun design was complete in 1940, but had to wait a full year before carriages were complete. Part of this delay was due to the fact that British command was shitting bricks over the fall of France, and preferred to have more crappy 2 pounders than fewer ok 6 pounders.

By May 1942, the British were in a stable enough position that they could finally produce the 6 pounder properly, and it started to see widespread service. Gun shields were issued, though apparently they were rarely used due to the sheer adamantium balls of the British artillerymen. Tanks that were equipped with the QF-6 pounder include early Churchill variants, the Valentine, the Crusader MK III, and the Cromwell and Cavalier tanks, as well as the AEC armored car. The weapon served the British for 20 years after the war, and was even adopted by the Navy on their lighter ships.

The Americans designated it the M1, and only considered it an interim solution, but ultimately the M1 became so widely adopted that by 1944 there was a widespread shortage of HE ammunition among the Americans. The 82nd and 101st airborne regiments used the M1 in combat and found it excellent, but complained about the lack of HE ammo during their landings in Normandy.

The M1 saw valuable service in helping throw back the German counteroffensive at the Hedgerows by being an effective weapon to ambush the side armor of Panther tanks with, and while it performed poorly on the offensive, it was one of the factors that kept the allies on the beaches.

The M1 was phased out of service shortly after the war, completing its service in the US military.