US 155mm

The 155mm cannon, commonly known as the ""Long Tom" was a Howitzer in use during the Second World War. It later became the primary armament of the M12 Gun Motor Carriage.

IRL

The 155 mm gun M1 howitzer, also known as the Long Tom started as a post World War I project in an attempt to add extra firepower to the US arsenal in preparation for a future conflict. Primarily, it was to be based on the French Canon de 155mm GPF, an effective weapon that saw service primarily as a coastal defence gun for allied forces across the world. It especially saw use in the Pacific, where they were expected to hold off melee oriented maniacs wielding shitty Katannas.

However, the Great Depression, combined with a general lack of caring about what the hell the Europeans did to each other resulted in a carriage for the weapon being delayed until 1938, when Roosevelt decided to act on his hate boner for Fascists and funded the military. In 1942, the M4 Tractor was introduced in order to ease transport of the weapon, along with transport of other US field guns.

The gun served across all theaters against all enemies of the United States, earning particular distinction in Europe, where 155mm of death was plenty enough to deal with the Tigers and Panthers that Infantry were too afraid to fight. The gun was even mounted to an M3 Lee Chassis and designated the M12 in order to provide a highly mobile critical existence destroying platform.

Based off experiments with the M12, a new version using the M4 Chassis was created post war and saw service in Korea, called the M40.

The original 155 mm gun M1 stopped being used around... well fuck if we know. Wiki says the 155 mm gun M1/Long Tom stopped being used after the Korean War but the 155 mm howitzer M1/M114 is still in use by some countries. What we're saying is the US army loves the 155mm shell and naming howitzer M1 and has made too many guns using that caliber and number. The latest generation of the Long Tom's grandchildren are (at time of writing) currently seeing action in Ukraine to great effect.