it relates to the total length of the weapon but I dont think there is a minimum barrel length rather the minimum lenght applies to whether or not the weapon has a removable stock, such as a take down rifle, if its under a certain length it is classed a pistol, air pistols are limited to 6ft/lbs, so if you have a rifle that can still be fired with the stock removed or if its under a certain length you have an over the legal limit pistol