4/10 ths of an inch: There are 25.4 mm in one inch. That means 10 mm is 4/10 ths of an inch (10 divided by 25.4 = 0.39). It is a hair more than three-eighths of an inch, because 3/8 ths is 0.375 inches. If one is familiar with pistol cartridges, 10 mm is slightly wider than the DIAMETER of a 9mm bullet or of a .380 bullet.
Answered 9/5/2016
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