JACKSONVILLE, Fla. -

Obviously guns can't work without ammunition, or bullets. There are countless categories and types of ammo, and it can get complicated.

To help dig through the complexities, we enlisted the expertise of forensic expert and former Jacksonville Sheriff's Office Crime Scene Detective Michael Knox.

He shows how it consists of the primer in the back of the cartridge, the case where the gun powder is and the actual bullet or projectile.

"The primer discharges and then that causes the propellant to discharge and that causes a buildup in pressure and then that buildup in pressure sensor the bullet out of the muzzle of the firearm," said Knox.

Ammunition is broken down into many different categories. There are shotgun shells, which have pellets inside, and there's ammo for rifles and handguns. Rifle ammo is usually longer.

Those two types can have several types of tips, but the most common are hollow-point and full-metal jacket. Full-metal jacket bullets have a pointed metal tip, they usually travel faster and farther after impact.

"This round could penetrate through a person and then continue on and still have enough velocity to hit somebody behind them," Knox explained.

Knox explains that a hollow-point round expands when it strikes tissue.

"That expansion does two things: It does cause more damage so it is more likely to cause injury," Knox said.

Knox says hollow-points are most often used by police and those who want their weapons for self defense: They stop the threat and they're less likely to travel through a person and hurt someone else.

Knox now makes a living reconstructing crime scenes and trying to solve them. He says technology is getting better, so officers can find bullets or casings at a crime scene and track down the shooter. Bullets that are fired have a unique fingerprint.

"There's a transfer of impressions from inside the barrel of the firearm to the outside of the bullet," Knox said.

Ammo is sold all over. You don't need any kind of license to get it, but must be 18 years or older for shotgun and rifle bullets, and 21 and older to buy for handguns.

Prices usually range from pennies a shot, to several dollars a round, often depending on supply and demand. Different guns take different calibers, or diameter. Some are measured as millimeters, others as fractions of an inch.

Shelves were barren across the country in recent years -- many speculate because millions of people bought guns and stockpiled ammo in fear of new, stricter gun laws.

"It's very hard to get ahold of .22s," said Anthony Testa, co-owner of Yulee Pawn and Collectibles.

Testa says they saw the rush, but things are starting to level out and bullets are back on the shelves, albeit sometimes for much higher prices thanks to the manufacturers.

Our experts stress that you need to be careful with bullets and casings just like the actual guns. Don't leave them out and don't leave them places where kids can get them. The best idea is to keep them locked away in a safe.