Why does a gun recoils on firing?

Answer 1

It happens because of Newton's 3rd law of motion: Every action has an equal and opposite reaction. The reason that the "kick" is so variable based on gun type is that the force involved is controlled by the formula F=MA or Force=Mass times Acceleration (AKA Newton's 2nd law).

If you have a .22 caliber long rifle round you are looking at perhaps 40 grains of lead (1 grain is about 2 thousandths of an Oz.) and it is leaving the barrel at perhaps 1100 feet per second (FPS). In other words, the bullet went from 0 FPS to 1100 FPS in the fraction of a second that it took to go down the length of the barrel. I won't bore you with the math but that movement represents an acceleration. When you multiply that acceleration times the mass of the bullet (40 grains) it tells you what the force was that caused that acceleration. That force is what you feel in the gun recoil.

A "30 odd six" (30.06) rifle uses a much bigger chunk of lead for the bullet - perhaps 150 grains or more and it flies out of the barrel at something like 3000 FPS. As a result, the force is much higher (still because of F=MA) and thus so is the recoil/"kick".
That only happens if something with mass shoots out of the front end, such as a bullet or some gases. When that happens, the stuff that shoots forward carries forward momentum, so something has to carry backward momentum in order to keep the sum at zero. That's usually the gun itself.

Answer 2

A gun recoils upon firing due to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. When a bullet is fired, it creates a force that propels it forward, while simultaneously creating an equal force that pushes the gun backward. The recoil is a result of this backward force acting on the gun.