The propellant chemicals in a handgun cartridge are not designed to explode suddenly, all at once: that would blow the whole gun open and very likely kill the person firing it. Instead, they are supposed to start burning relatively slowly, through a process called Deflagration, so the cartridge moves off smoothly down the gun. They burn faster as the bullet accelerates down the barrel, giving it a maximum “kicking” force just as it comes out of the end. As the cartridge emerges, the whole gun recoils because of a basic law of physics called “action and reaction”. When the gas from the explosion shoots the bullet forwards with force, the whole gun jolts backwards with an equal force in the opposite direction. The explosion that fires a bullet happens in the confined space of the gun barrel. As the bullet flies out of the gun, the pressure of the explosion is suddenly released. That’s what makes a gun go BANG! Some bullets also make noise because they go so quickly. The fastest bullets travel at around 3000 km/h, about three times the speed of sound. Like a supersonic jet fighter, these bullets make shock waves as they roar through the air.