Franchi,
Take a 1911 auto and slip a wooden dowel that fits the bore down the barrel. It'll show you that indeed an auto also has the barrel pointed downward from the sighting plane. The barrel in a 1911 is not parallel with the slide or its rails when in battery. The back of the barrel cams up into the lugs when in battery and drops down to approximately parallel with the slide when the slide is retracted.
As I said earlier, if you doubt this, set your favorite sxs barrels on a table or something, with the rib sighting plane pointed at a selected target. Then look down each bore.
To explain this, I think it's best stated as "for every action, there's an opposite and equal reaction". If a mass (projectile/shot) is accelerated in one direction, the gun encounters a equal force in the opposite direction. Like Miller said, as soon as the projectile starts to move, there is force pushing on the gun in the opposite direction and causing movement.
