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.
5.56 mm
9 mm
MM is a measurement in the metric system. Caliber is a measurement in inches.
compare it with cm .....mm x 10 cm
.30 caliber.
5.56 x 45 mm