"IF" you are speaking of true diameter then 18mm = .709". 13 gauge = .710", close enough I'd say 13 gauge.
Note that neither the 14mm or 12mm are true sizes for 32 gauge or .410 but are simply "Names". 14mm = .551" (28ga = .550) while a 32 gauge is .526".
12mm = .472" with a .410 being of course .410", not a gauge at all. There have been as I recall some actual 18 gauge guns made whose bores would be a nominal .637"

Miller/TN
I Didn't Say Everything I Said, Yogi Berra