Shotguns are most commonly classified by two main concepts: gauge (the bore size) and action (how the gun operates).
- Gauge: This is the traditional way shotguns are grouped. It refers to the internal diameter of the bore and, historically, how many lead balls of that bore would weigh one pound. Common gauges include 12-gauge, 20-gauge, 16-gauge, 28-gauge, and so on, with smaller gauge numbers corresponding to larger bores. The unique exception is the .410 bore, which is not a true gauge but is treated as a distinct category. Shells are manufactured to match the gauge of the corresponding gun.
- Action: Shotguns are also categorized by their operating mechanism, or action. The most common types are break-action (single or double barrel), pump-action (slide-action), and semi-automatic. Each action type has its own practical advantages in terms of handling, rate of fire, and reliability.
- Other notes: While bore diameter (gauge) is a primary classification, some discussions also reference barrel length, choke (the constriction at the muzzle that affects spread), and overall configuration (single-shot, over/under, side-by-side, etc.). However, gauge and action remain the principal axes for classifying most shotguns.
Direct answer: Shotguns are classified primarily by gauge (bore size) and by action (the mechanism that cycles shells), with gauge indicating the shell size they use and action describing how the gun loads and shoots.
