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From left: .50 BMG, .300 Win Mag, .308 Winchester, 7.62×39mm, 5.56×45mm NATO, .22LR (Note that the 5.56x45mm NATO roond an the .22LR roond hae the same diameter bullets but very different cartridges)

In guns, pairteecularly fireairms, calibre is the approximate internal diameter o the barrel, or the diameter o the projectile it fires, in hunderts or sometimes thoosandts o an inch.