A base example is: a 9mm parabellum bullet is .356 caliber.
Roughly .275 or .28 caliber. Take MM divided by 25.4 (mm/inch) and you get caliber in hundredths of an inch.
9 caliber is 0.09 inch, smaller than any standard ammunition made. 0.09 inch = 2.286 mm. Perhaps you were thinking "What caliber is 9 mm?". 9 mm = 0.354 inch = 35.4 caliber.
.68 caliber is ABOUT 17mm.
The question can't be answered since you didn't supply a caliber or mm measurement.
No! 9mm = .35 caliber - .354 caliber!
7.62 mm (or 30 caliber) and 5.56 mm (or 223 caliber) are common rifle cartridges. Pistols, machine guns, and artillery will of course be different from these.
9 mm
5.56 mm
compare it with cm .....mm x 10 cm
MM is a measurement in the metric system. Caliber is a measurement in inches.
.30 caliber.
5.56 x 45 mm