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--adj.

1. of or relating to the bore of a gun (or its ammunition) that measures twenty-two hundredths of an inch in diameter.
  • "A .22 caliber pistol."
2. of or relating to the bore of a gun (or its ammunition) that measures twenty-two hundredths of an inch in diameter.
  • "A .22 caliber pistol."

The 5.6 mm caliber or .22 caliber, is a small, extremely common size of ammunition, fitted to firearms with a bore diameter of 5.6 mm.