1. aperture; bore; calibre; caliber
Wikipedia definition
2. CaliberIn guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it. In a rifled barrel, the distance is measured between opposing lands or grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere. It is important to performance that a bullet should closely match the groove diameter of a barrel to ensure a good seal.
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