Why are British guns usually expressed in pounds in World War II, such as "17 pound anti-tank guns"?

Yes, in the era of old-fashioned front-loaded smoothbore guns, it was customary to name guns by the weight of shells, because the early processing technology of gun bore was not accurate, and the front-loaded Wei-bore guns mainly fired spherical shells, so it was more convenient to name guns by the weight of shells than by the caliber. However, in the era of rifled guns, because cylindrical shells are used, there is no necessary connection between weight and caliber, so caliber is usually used to indicate the size of guns. As an old imperialist country, Britain naturally retains many traditional habits in the military. During World War II, the British army still used pounds to represent small and medium caliber guns, and large caliber guns were changed to inches. After the end of World War II, Britain lost its leading position in the world and its weapons technology was no longer in the lead. NATO forces unified the metric representation used by the US military, and expressed caliber in millimeters.