Gauge
Shotgun bore measurement; lower numbers indicate larger bores.
Gauge is an old measurement: the number of pure-lead balls of the bore’s diameter that weigh one pound. So a 12-gauge has a larger bore than a 20-gauge (it takes only 12 balls to make a pound at that diameter; you’d need 20 smaller balls). The exception is .410 — that’s a true caliber measurement (.410 inches).