If the barrel is in an absolutely horizontal state, when the bullet moves in the barrel without a muzzle, the thrust of the gunpowder gas is accelerated in the horizontal direction, and the vertical direction is again subjected to the gravity of the earth, but at this time there is a barrel wall. The support, the vertical force of gravity and the elastic force provided by the barrel wall balance each other, so it can be ignored. When the bullet flies out of the muzzle, the bullet flies by inertia and no longer accelerates; in the vertical direction, it is subjected to gravity, and there is resistance of the air, then the trajectory of the bullet is roughly a flat throwing motion.

So for the gun, it means that as soon as the bullet comes out of the muzzle, it begins to fall, lower than the horizontal line of sight, the bullet will definitely fall below the aiming point, and the farther away, the greater the amount of the lower part, which is obviously Can't hit the target. Therefore, when shooting, the barrel is actually lifted up slightly. The trajectory after the bullet flies out of the muzzle is not a straight line, but a parabola. Only this slight lift is very small, you don't feel it.