Short answers to the questions we hear most. New to ammo? Read this, then head back to Where to Start.
What’s in a cartridge?
- Bullet (projectile): the part that leaves the barrel. Usually lead, often with a copper jacket. Shotguns fire shot or slugs instead.
- Case: holds everything together. Usually brass, sometimes steel or aluminum. Very old ammo used paper or foil.
- Powder (propellant): burns fast and turns into expanding gas.
- Primer: a small charge that ignites the powder when the firing pin strikes it.
How does ammo fire?
- The firing pin strikes the primer.
- The primer ignites the powder.
- Burning powder creates expanding gas and pressure.
- Pressure pushes the bullet out of the case and down the barrel.
Centerfire vs rimfire
- Centerfire: used in most pistols, revolvers, rifles and shotguns. Higher pressure, and most cases can be reloaded.
- Rimfire: mostly .22 LR and other small calibers. Low pressure, inexpensive, and not reloadable. See Rimfire Ammo.
What does caliber mean?
Caliber is the bullet diameter, given in inches (.45) or millimeters (9mm). The full name, like 9mm Luger or .45 ACP, tells you the exact cartridge. Some share a diameter but are different lengths or pressures, so they are not interchangeable. One safe exception: a .357 Magnum revolver can also fire .38 Special, but not the other way around. More on Caliber.
Common calibers chart
| Caliber | Bullet diameter | Used in | Good for |
|---|---|---|---|
| .22 LR | .223 in | Rifle, pistol (rimfire) | Plinking, training, small game |
| 9mm Luger | .355 in | Pistol | Self-defense, range, duty |
| .380 ACP | .355 in | Pistol | Pocket pistols |
| .38 Special | .357 in | Revolver | Self-defense, light recoil |
| .357 Magnum | .357 in | Revolver, carbine | Defense, hunting, also fires .38 Special |
| .45 ACP | .451 in | Pistol | Self-defense, range |
| .44 Magnum | .429 in | Revolver, carbine | Hunting, big-bore fun |
| 5.56×45 / .223 | .224 in | Rifle (AR-15) | Range, defense, varmints |
| 7.62×39 | .311 in | Rifle (AK, SKS) | Range, deer-size game |
| .308 Win / 7.62×51 | .308 in | Rifle | Hunting, long range |
| 12 gauge | .729 in bore | Shotgun | Birds, deer, home defense |
What is the best self-defense round?
The one that feeds reliably in your gun and that you can shoot accurately. Pick a quality hollow point from a major maker, in a common caliber like 9mm, .38 Special, .357 Magnum or .45 ACP. Run a box or two through your gun before you carry it. See Bullet Types.
Is old ammo safe to shoot?
Factory ammo stored cool and dry can last for decades. Skip any rounds that are:
- Corroded, green or crusty
- Dented or with a loose or pushed-in bullet
- Wet, or with a damaged primer
Some old military surplus uses corrosive primers, so clean your gun the same day. If a shot sounds unusually weak, stop and make sure nothing is stuck in the barrel before firing again.
How should you store ammo?
- Cool, dry and out of direct sunlight. Sealed ammo cans with a desiccant pack work great.
- Keep it in original boxes or label it by caliber.
- Lock it up away from kids.
- Rotate stock: shoot the oldest first.
Quick answers
- What is a grain? A unit of bullet weight. 7,000 grains = 1 pound.
- What is +P? A higher-pressure load. Only use it if your gun is rated for it.
- FMJ or hollow point? FMJ for practice, hollow point for defense.
- Is steel-case ammo OK? Usually cheaper and fine in many guns, but some ranges ban it and it cannot be reloaded.
- Clip or magazine? A magazine has a spring and feeds rounds into the gun. A clip holds rounds for loading. See Magazines & Clips.
- Can I reload my own? Most centerfire brass, yes. See Reloading Ammo.
Always follow your firearm maker’s ammo guidance and check your state’s laws.