A firearm is a portable gun, typically consisting of a metal tube from which bullets or other projectiles are propelled by explosive force, usually produced by the combustion of gunpowder or other propellant. Firearms come in various types, including handguns, rifles, shotguns, and submachine guns, each designed for different purposes such as self-defense, hunting, sport shooting, or military applications. 

Photo shows a modern rifle with scope and a late 1800's vintage .30-40 Krag.

Caliber

The term "caliber" when referring to firearms and ammunition is a measure of the internal diameter of a gun barrel or the diameter of a bullet. It is usually expressed in inches or millimeters. The diameter of the bullet or the bore of the gun is a critical factor in determining the appropriate ammunition for a particular firearm.

For example, if a firearm has a caliber of .45 inches, it means that the internal diameter of the gun barrel is approximately 0.45 inches. In the context of ammunition, a bullet designed for a .45 caliber firearm would also have a diameter of around 0.45 inches.

It's important to note that caliber is just one aspect of the overall cartridge specification. The complete cartridge information typically includes the caliber, cartridge length, and sometimes additional information, such as the specific type of ammunition or load. Understanding the correct caliber is crucial for selecting the right ammunition for a firearm to ensure proper function and safety.

There are numerous bullet calibers used in firearms, and they vary widely depending on the type of firearm and its intended purpose. Here are some common bullet calibers:

• .22 LR (Long Rifle): Common for rimfire rifles and handguns, often used for plinking and small-game hunting.
• 9mm Parabellum (9mm Luger): Widely used for handguns, including many military and police firearms.
• .45 ACP (Automatic Colt Pistol): Commonly used in handguns, particularly in large-frame pistols.
• .380 ACP (9mm Short): Used in compact and subcompact handguns for self-defense.
• .40 S&W (Smith & Wesson): Popular for law enforcement and self-defense applications, often found in semi-automatic pistols.
• .38 Special: Common in revolvers, used for both self-defense and target shooting.
• .357 Magnum: Similar to .38 Special but more powerful, often used in revolvers.
• .223 Remington / 5.56x45mm NATO: Common rifle cartridges used in civilian and military rifles.
• .308 Winchester / 7.62x51mm NATO: Used in a variety of rifles, including hunting and military rifles.
• 12 gauge and 20 gauge: Common shotgun gauges, used for various applications such as hunting and sport shooting.

These are just a few examples, and there are many other calibers for different firearms and purposes. When selecting ammunition for a firearm, it's crucial to use the correct caliber and type specified by the firearm's manufacturer. Mixing incompatible calibers can be dangerous and may result in malfunctions or accidents.