12 mm or 12 mm gauge may refer to: Rail transport modelling TT gauge, 1:120 scale with rails 12 mm apart, representing standard gauge HOn3½ gauge, 1:87.1 scale with rails 12 mm apart, representing 3-foot-6-inch gauge H0m, 1:87 scale with rails 12 mm apart, representing metre gauge 00n3, 1:76 scale with rails 12 mm apart, representing 3-foot gauge 5.5 mm scale, 1:55 scale with rails 12 mm apart, representing narrow gauge On2 gauge, 1:48 scale with rails 12.7 mm apart, representing narrow gauge Firearms 12 mm caliber