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Meg is a 510 mm or 20 inch bombard.
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.