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Caliber is the measure of the bullet diameter. It can be expressed in inches or millimeters. 5.56mm is a caliber as is .223. It is just easier to say .23 caliber than it is to say 5.56mm caliber. Caliber is a notation that is not restricted to less than an inch. 40mm is a caliber that is greater than an inch.
So 4.2 caliber could be used to denote 4.2mm.
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