Choke measurement

[SWAMPUS]

Here's a poser for ya'll.I own 1 shotgun where choke designation is in (I think)in millimeters.Am looking at another.According to wikipedia 12 ga open is .750" or 18.5 mm.Near as I can tell, .1mm is .005".So-18.4 would be skeet,18.3 IC,etc.Does this sound right or am I off?BTW both bbls of my Jamart sxs are marked 18.3/18.4.I thought this meant one bbl 18.3 ,the other 18.4.Apparrantly not so.The Jamart I'm looking at-both bbls marked 18.2,so to my limited math both are choked MOD.If ya'll agree I'll probably buy it.What say you?

 

[oneounceload]

Here's a choke designation chart that is handy:
http://www.hallowellco.com/choke_chart.htm

Here is a MM to IN calculator:
http://www.metric-conversions.org/length/millimeters-to-inches.htm

Nominal 12 gauge barrel is .729; however it can range from .725 (18.4) to .750 (Kolar likes that one)

18.2 = .716. IF your bores are .729, that would mean a choke of .013, or IC in 12 gauge.
18.4 is right at .725 - a nominal, but a little tight 12 bore