Another thing to keep in mind: the numerical designation of a
given caliber often bears little resemblance to the actual diameter
of the projectile.
For example, the 380 ACP, 9mm, 357 Sig and 38 Super all have the
same projectile diameter of 0.355". Ditto with the 40 S&W and
10mm Auto (0.401" ), and the 38 Special and 357 Magnum (0.357" ). A
caliber's official numeric designation is more often the result of
a company's desire to differentiate a particular cartridge from
others sharing the same projectile diameter, for marketing
purposes.
Though the diameters may be identical, the projectiles can and do
vary in length, weight and profile. The 380 ACP, for example, is
usually offered in projectile weights between 90 and 100 grains
(where 7000 grains = 1 pound), and the 357 Magnum is available from
110 to 180 grains.
