Caliber
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In guns, particularly firearms, but not artillery, where a different definition may apply, caliber is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bo
7.92 mm caliber rifle barrel (Mauser m/96)
Rifle cartridges: from left: 50 BMG • 300 Win Mag • 308 Winchester • 7.62 × 39 mm • 5.56 × 45 mm NATO • 22 LR
A 45 ACP hollowpoint (Federal HST) with two 22 LR cartridges for comparison
Gauge (firearms)
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The gauge of a firearm is a unit of measurement used to express the inner diameter and other necessary parameters to define in general a smoothbore barrel.
Caliber
…a different definition may apply, caliber (or calibre; sometimes abbreviated as "cal") is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bore is measured and whether the finished bore matches that specification. It is measured in inches or in millimeters…
From left to right; a .45 ACP cartridge, a .410 bore shotshell, a 20-gauge shotshell, and a 12-gauge shotshell
Portrait of Frederick Courteney Selous with his 4 bore single-shot Boer rifle and African hunting regalia, 1876
Left to right: .410 bore, 28-gauge, 20-gauge, and 12-gauge shotgun shells
Garden gun calibers: 9mm Flobert shot, 9mm Flobert shot, .22 Long Rifle shot, .22 Long Rifle, .22 Long Rifle shot, .22 CB Short, and 9mm Flobert BB cap