I was on a firearms forum and a guy said something along the lines of this. When you're handloading your own cartridges, if you use a mild powder load (less powder) in a handgun versus a hot load (more powder), you would think that shifts the point of impact of the bullet on the target lower since the bullet will be going slower. But in fact, the point of impact can increase a few inches at 20 yards because the slower bullet creats more muzzle rise from the recoil of the bullet as it passes through the barrel slower. Muzzle rise when shooting is caused by a moment of angle created from the difference of the point of contact between your hand and the gun and the bore axis not being in the same horizontal plane. The bore axis is always higher than your hand grip which makes the muzzle rotate upwards around the center of your hand grip. Recoil in firearms is momentum. I always assumed that there is zero recoil from a bullet until it and the powder gasses exit the barrel. My assumption is that there is no change in momentum in a closed system, and that momentum only changes when it exits the barrel making it an open system. The momentum of the bullet is definitely changing while it's accelerating in the barrel, but does that really create recoil before it exits the barrel or does the recoil happen after it leaves. In a semi-automatic handgun, the barrel and slide remain locked and do not move until some point after the bullet has left the barrel.