As mentioned earlier, the force which acts on the rear of the bullet to propel it forwards is also exerted on the base of the cartridge case to move the gun backwards. The formula for this is as mentioned before:
It is this force which causes the weapon to recoil. This force not only drives the gun to the rear, but because the barrel is situated above the hand, and therefore above the rotational axis of the wrist, it also rotates the gun in an upwards direction. As the bullet is travelling down the bore during the period in which the barrel is lifting, it will strike the target above the point at which the barrel was pointed when the trigger was pulled.
As was seen earlier, the actual time the bullet is in the barrel is very short, and for the worked example of a 0.45" calibre self-loading pistol, it is only 0.00102 s. During this very short period of time, the muzzle lifts above its point of aim only a fraction of an inch. This does, however, have a pronounced effect on the striking point of the bullet as only a minute (¿0 of a degree) of barrel lift will change the impact point by 1.047 in. at 100 yd. To compensate for this, the sights of a weapon are set, or regulated, at the factory. The sights are regulated for a certain weight bullet travelling at a certain velocity. If a heavier bullet is used, the bullet will remain in the bore longer. The longer the bullet is in the bore, the more time the recoil has an effect on the hand and the greater the degree of barrel rotation. Thus, a heavier bullet will strike above the point of aim and a lighter one below. This is exactly the opposite of what common sense would indicate.
To give an example of the magnitude of barrel lift: when a standard military P14 0.303 rifle is fired with standard military ammunition, weapon recoil will cause the barrel to rise by 0.1" between the time the trigger is pulled and the bullet leaves the barrel.
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