A length of six millimeters is equal to .236 inches. In order to convert a measurement from millimeters to inches, millimeters should be divided by 25.4, the number of millimeters in one inch. Six millimeters is close to one-quarter inch, approximately .014 inches short of that measurement.
Several types of ammunition are described as 6 millimeter calibers. A caliber measures the internal diameter of a firearm barrel and the diameter of the bullet that travels down said barrel. Although the various ammunition are described as being 6 millimeters, the diameter of the bullets of these rounds are generally 6.17 millimeters and above.