As title suggests I need to know why it is important for guns to have longer barrels and larger caliber (approximate internal diameter of the barrel) or better say why for example great powers espcially during WWII were in race to acquire guns (or tank guns) with big caliber & longer barrel??? I've heard somewhere that longer barrel helps the gas to propel trajectory for longer period of time thus giving it higher velocity when it exits from the end of the gun barrel, is it true? Also I know bigger caliber might contribute to explosive in at least HE shells but have no idea how it can contribute to anti-tank aspect of the shots?