When a gun trigger is pulled, the primer is struck, igniting the propellant. This causes a rapid expansion of gas, which forces the bullet forward and ejects the casing.

As a bullet begins to leave a gun, it is subjected to a variety of forces that determine trajectory. Gravity, drag, ballistic coefficient and velocity are among the most familiar to shooters, but there are dozens more that, all combined, make a bullseye even more spectacular.

When a bullet is fired from a gun, total momentum before is zero since nothing is moving. After firing the bullet there is a momentum in the forward direction. The gun must therefore have the same magnitude of momentum but in the opposite direction so that they cancel each other out leaving the total momentum still equal to zero.