Many people don't know what a bullet is, other than it's something that a gun shoots. This page is intended to clear up some common questions about bullets and ammunition, then!
First, let's begin with some basic terminology.
This entire object is referred to as a cartridge, or a round.
1) This is the primer of the cartridge. When the trigger is pulled, the firing pin or the striker impacts this, which sets off a small explosion. This small explosion, which in turn sets off the powder.
2) This is the powder. The burning powder expands very rapidly, producing hot gasses that makes the bullet fly down the barrel and into the target.
3) The brass shell around the cartridge is called the casing. This holds everything together.
4) The end of the round is the actual bullet. People commonly refer to the cartridge as a bullet, but this is technically false. Only the part that is propelled out of the barrel is a bullet.
There are several common types of bullets, each with variations on them as produced by different manufacturers. Here are the most common types:
Full Metal Jacket
Jacketed Hollow Point
Jacketed Soft Point
Lead round Nose
Semi Wad Cutter
Lead Wad Cutter
There are several variations on each type of bullet. For instance Federal markets their famous Hydra-Shok JHP's, with a post in the center of the hollow in order to help the bullet expand and penetrate deeper into the body. Also, Glaser, Magsafe, and several other companies make 'pre-fragmented' safety bullets that are supposed to lessen the chance of richochets and over-penetration. The lead center of the bullet is replaced by #12 bird shot, so that when the bullet strike a target, these pellets expand out into the body, but wont penetrate walls. However, these bullets are generally very expensive, and poor stoppers.
One bullet type that I did not put up there is Armor-Piercing. Armor Piercing is different from regular bullets in that it has a penetrator inside the bullet that is made of a metal denser than lead - steel and tungsten are common penetrator types. Armor Piercing bullets are generally illegal to import into the U.S., except for certain types, including the SS109 AP 5.56 bullet.
People often talk about 'Cop Killer Teflon Coated Dum-dum bullets' or some such nonsense. Teflon-coated bullets are not inherently more dangerous than a normal bullet. It was thought that coating a bullet in Teflon may allow it to penetrate armor better. However, when laboratory tests were conducted upon these bullets, they performed no better than a normal bullet in penetrating armor, walls, glass, etc. Nowadays, some bullets are still Teflon coated, but instead of aiming to increase armor penetration, the Teflon coating is supposed to reduce friction between the bullet and the barrel, supposedly giving a better muzzle velocity. Dum-dum bullets are supposed to be bullets that have a X carved into the tip, and are supposed to be vicious against human targets. Well, again, this is false. Dum-Dum bullets don't expand, and if they did, they certainly wouldn't do so as neatly as a normal JHP type of bullet. In Lethal Weapon, Mel Gibson used some special Dum-dum bullet to blow a hole the size of a softball in a bullet proof vest. This is entirely false - heck, you could probably see the explosive charge set on the vest by the SFX guys if you look close enough. So if anyone claims to have some special armor piercing Teflon bullets, or if your friends are trying to carve up bullets so that they can blow stuff up better, tell them to save their money and stop wasting their time.
Jacketed Hollow Points are generally the best choice against an unarmored human target. When they impact, the bullet 'mushrooms,' often expanding up to 60 to 80 percent more than the nominal diameter of the bullet. This creates a wider wound channel, causing more blood loos and faster incapacitation. Full Metal Jacket rounds tend to simply plow through a target, creating a smaller, narrower wound channel, thus requiring more shots to stop an attacker. However, they do penetrate armor better than JHP's - but not as well as true AP bullets. Wadcutter type bullets are generally used for target practice, since they leave a neat, round hole in paper targets.
According to the Geneva and Hauge conventions, it is illegal to use JHP type bullets in combat. However, those conventions only apply in times of full-out war. Counter-terrorists are not engaged in warfare, and so, they are allowed to use hollow points against terrorists.