What’s one millimeter more? 9mm vs 10mm… Number’s wise, it might seem minor at best, like having $9 vs $10, but as far as ammunition calibers are concerned, that increase of one millimeter makes all the difference.

The average grain weight of a 9mm bullet is around 115 grains, and the average 10mm is around 180 grains. Making the 10mm bullet 63% heavier of a round over the 9mm, that one digit millimeter difference, is really the equivalent of 63% more mass.