During a recent self-defense class, a female student asked me "Why does the gun go bang when I pull the trigger."One of the other students, confident in his own understanding of such complexities, offered the advice that "it’s the explosion of the powder inside the ‘bullet’". This, I noted, came from the same guy who had spent most of the morning lecture arguing with our instructors about one point or another, and, the same guy who came to class with a .25 autoloader, his "carry gun", and a short-barreled .44 magnum, his "home defense gun".
Sigh . . . I guess every instructor gets to deal with one of these sorts every once in a while. I was gentle, but also had to let him know that he was off-base in a number of directions. I gave the questioning student a thumbnail sketch of the answer I suspected she was looking for. Powder, gas, etc. Then I gave her a quick survey, and asked, "What do you do for work?" "Middle-school science teacher", was the reply. I thought for a second, then told her I would e-mail her a more thorough explanation of the process after class.
So, this is what I sent her:
You will remember from the class lecture that the centerfire pistol cartridge is constructed from four basic components: Cartridge case, primer, powder charge and bullet. When the hammer of the handgun falls, it strike the firing pin driving it into the center of the primer of the cartridge seated in the chamber. This impact causes the primer to flash, igniting the powder charge, which burns rapidly creating gases, the pressure of which forces the bullet out of the casing and down the barrel, the path of least resistance. The rapid expansion of gases does not "explode", but when the bullets leaves the muzzle of the handgun, the superheated gases impact with the cooler air outside the barrel, creating a shockwave which your ears hear as the "bang". The handgun recoils as the bullet leaves the muzzle, but, not as a result of an explosion. And I think we have made it pretty clear, "bullets" don’t contain any powder.
But, since you took the time to ask, here is hopefully a better explanation of "why" and "how all this happens.
The primer flashes when struck because it is a metal cup with approximately one grain of a (usually) pink impact sensitive explosive compound packed into it. At the open end of the primer is seated a brass-colored metal device, shaped very much like a three bladed fan, which is known as the "anvil". The anvil holds the shock sensitive explosive against the bottom of the primer and also provide a "flash hole" for the flame to ignite the gun powder. The sudden impact of the firing pin on the back of the primer dents the back of the cup inward and squeezes the explosive between the anvil and the wall of the cup. This impact causes the explosive to detonate shooting a small flame forward through the flash hole igniting the powder inside the bullet casing.
This explosive compound in the primer is usually Lead Styphnate, although other compounds have been and may be used. Lead styphnate is used because in addition to its high impact sensitivity, it does not react with metals and is less sensitive to shock and friction than mercury fulminate which has been used for this purpose in the past.
When the flash from the primer ignites the powder inside the cartridge case, it begins to burn very rapidly. This rapidly burning powder creates gases which heat to very high temperatures. The heating of the gases increases its volume and, therefore, increases the pressure inside the casing, forcing the bullet out of the casing and into the bore of the barrel. Since the bullet is slightly larger than the inside diameter of the bore, the surface of the bullet is forced into the grooves of the rifling inside the barrel. This expansion is known as "obturation" and it seals the barrel so that the still expanding gases are trapped behind the bullet and must force it down the barrel and then, upon leaving the barrel at the muzzle, downrange toward the target.
The powder which makes all this happen is known as "smokeless powder" and it has been in use since the mid 1880's. Modern smokeless powders are nitrocellulose-based propellants. These are either single-base, using only nitrocellulose, or double-base which use nitrocellulose and also nitroglycerine. There are a wide variety of powders for different applications. Much of this variation is in the burning rate of the powder.
Pistol powders are designed for a rapid burn due to the short barrel length involved. The powder burns as the bullet travels down the barrel and out the muzzle, accelerating as it moves through the bore. It reaches maximum velocity as it leaves the barrel which causes the handgun to recoil because of Newton’s third law of motion which you teach to your sudents as "action and reaction". So, when the gas from the burning powder forces the bullet forward, the handgun recoils backward with an equal amount of force.
As the bullet leaves the muzzle, the super-heated gases behind it are also released, colliding with the cooler ambient air ahead of the muzzle. This collision of super-heated gas and cooler air results in a shockwave, very similar to the shock wave created by air suddenly being superheated by lightening (60,000 F.). This we hear as thunder. So, in a very real sense, if somebody asks you why a gun goes bang when you shoot it, you can now smile and say, "its just thunder".
So, now, if nothing else, you clearly know its not a case of "exploding bullets".