I think it’s the ‘equalizer’ aspect. A knight would have to train for years to be proficient in swordplay, horse riding, and usage of a lance. Longbowman had to train from childhood. A wizard has to study magic for years.
A peasant could pick up a gun and theoretically kill any of them.
Americans in general are also more familiar with guns than swords or bows, so the “effectiveness” is more intuitive for our minds.
Well, if it’s a shaped explosion round, aka HEAT, then it ‘forges’ a jet of molten copper upon detonation, that is what ‘cuts’ though armor plate. Pedantic, yes. Technically not forged by human hands, also yes.