12 mm
12 mm is a cartridge or ammunition calibre.
In detail
12 mm is a cartridge or ammunition calibre.
Sources
Each source is named with the standing the house places on it, so the evidence can be weighed rather than taken on trust.
reference work · Reputable secondary · Wikipedia
“12 mm caliber”, English Wikipedia, consulted as further reading
Consult the sourcereference work · Reputable secondary · Wikidata
Wikidata, structured authority record Q38194: 12 mm
Consult the source