A gun is a weapon that uses the force of an explosive propellant to project a missile. Guns or firearms are classified by the diameter of the barrel opening.  This is known as the calibre of the gun.  Anything with a calibre up to and including .60 calibre(0.6 inches) is known as a firearm.

The precise origin of the gun is unknown, although they were in use by the early 14th century and were commonplace in Europe by mid-century.  These early guns were nothing more than large caliber cylinders of wrought iron or cast bronze, closed at one end and loaded by placing gunpowder and projectile in the muzzle, or open end.

Nowadays firearms are a little more sophisticated. However, the physics behind all guns remains the same.  Weapons such as cannons, shotguns, and rifles,  work on the basic idea of conservation of momentum and the change in energy from potential to kinetic.

When the trigger is pulled the hammer hits the firing pin.  The firing pin then hits the primer which causes the powder to burn hence producing lots of gases.  This causes the volume behind the bullet to fill with extremely high-pressure gas.  The gas pushes on every surface it encounters, including the bullet in front of it and the base of the gun barrel behind it. 

The increase in pressure caused by the gases causes the bullet to be forced into the barrel hence causing the bullet to come out of the muzzle at very high speeds. Once the bullet is fired, it remains in motion from its momentum.  The momentum will carry the bullet until it strikes an object or gravity pulls the bullet towards the earth.

Firearms change potential chemical energy into kinetic energy in the actual firing of the gun.  Many people do not realize that the force imparted by accelerating the bullet is not the only force acting on the gun, or the shooter.  Grains of burned gun powder is sent out the muzzle at high velocity.  When the trigger is pulled, the hammer strikes a small charge at the end of the shell, the ammunition. 

This charge ignites black gun powder packed behind the lead ball bearings.  When the black gun powder burns, it produces gas that rapidly expands with the burning of more black gun powder.  High-pressure gases exert forces on the back of the bullet and on the gun.  The only way for the gas to escape is to push the bullet out of its way through the end of the barrel.  This is how a bullet is fired from a gun.

Conservation of momentum is the law that is held true when the gun is fired and a “kick” is felt.  When a bullet is fired from a gun, the total momentum before is zero since nothing is moving.  After firing the bullet there is 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.  For this reason, the gun must have a recoil velocity after the bullet is fired (i.e. the gun ‘jumps’ backward and a ‘kick’ is felt).

As the bullet is propelled through the barrel, it gains momentum.  In order for the entire system of the gun and the ammunition to have equal momentum, the gun must gain momentum in the opposite direction from the bullet.  Momentum is a vector quantity, having both a direction and a magnitude

The faster an object is moving or the more mass it has, the more momentum it has in the direction of its motion(momentum = mass x  velocity).  Because momentum is a conserved quantity, it cannot be created or destroyed (momentum before = momentum after).  It can only be transferred between objects. Momentum is conserved because of Newton’s third law of motion.

When one object exerts a force on a second object for a certain amount of time, the second object exerts an equal but oppositely directed force on the first object for exactly the same amount of time.  The momentum lost by the first object is exactly equal to the momentum gained by the second object.  Momentum is transferred from the first object to the second object.

In this case, if a gun exerts a force on a bullet when firing it forward then the bullet will exert an equal force in the opposite direction on the gun causing it to move backward or recoil.  Although the action and reaction forces are equal in size the effect on the gun and the bullet are not the same since the mass of the gun is far greater than the mass of the bullet.  The acceleration of the bullet while moving along the gun barrel would be much greater than the acceleration of the gun(acceleration = force mass).

The conservation of momentum is also demonstrated when the bullet hits an object.  The object that it strikes absorbs the kinetic energy, energy from motion and momentum.  If the force of momentum from the bullet is great enough to overcome the mass of the object, the target will be moved along the same vector as the bullet.

To increase the accuracy of the flight of the bullet, a technique called rifling can be used.  Rifling is where the barrel of the gun and or the bullet is creased with spiral grooves that allow air to pass through.  When the bullet is fired, the air passes through these curved grooves and spins the bullet.  This spinning action allows the bullet to cut through the air more efficiently and fly on a truer course, thus stabilizing its trajectory.


  1. Recoil is the more correct term instead of kick. Recoil is the vertical and horizontal blow back movement of a gun when discharged. The word “kick” is more commonly said when talking about a rifle because rifles have more horizontal recoil than vertical recoil. Second, black powder propellants are only used in percussion cap, flintlock, wheel lock and matchlock guns. All modern bullets use nitrocellulose as the single base powder. Not all bullets are made out of just lead. Many projectiles are, yes, made out of lead (usually the core of the projectile is made out of lead because lead is very strong), but the coating is usually copper and a bit of zinc. No one refers a bullet as a missile. They’re usually just called bullets and projectiles.

    • The term ‘kick’ is a common term when referring to the recoil of any firearm, but I personally think that it’s just because people either don’t know the proper term or don’t want to use it. Also, ‘firearm’ doesn’t pistols, rifles, and shotguns. It also refers to cannons. They use 3 different kinds of ammunition: bullets, shot, and cannonballs. To make it easier, they can be generalized by one term: ‘missile,’ but ‘projectile’ also works, really just depends on who you are.

    • a missile is an object which is forcibly propelled at a target, either by hand or from a mechanical weapon. Bullets are missiles.

        • So are missiles, but I can see your point, Today, a bullet is ammunition in a case that uses gunpowder to project itself over large distances, while missiles are much bigger and use liquid fuel instead, so while one CAN say that both are bullets, they are separate.

          P.S. if I am incorrect, feel free to correct me, as it would be a cool learning experience to know more about missiles and bullets.

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