Use a "Metric Converter" ... convert millimetres to inches, 4 decimal place answer. Subtract choke from bore in inches.
"Typical" 12's run anywhere from 0.725" (older tight European doubles) to 0.745" (modern "back-bored" guns)
If one consider's an average 12 gauge Full Choke to be around 0.040" , then 3/4 Choke (or Improved Modified) is about 0.030", Modified, or 1/2 choke is conventionally about 0.020" and 1/4 choke or Improved Cylinder is about
0.010". Cylinder is bore diameter, all the way through. Skeet chokes generally run 0.003" to 0.007" and Light Modified, popular with the Sporting & Upland crowd, is about 0.015" constriction. These numbers are only averages and can and do vary a few thousandths of an inch from manufacturer to manufacturer. Individual barrels on some double guns will vary in bore size one or two thousandths of an inch, as can identical barrels from one pump or auto-loader to the next.
And although the chokes may be marked one thing, they may measure something else entirely, and/or perform quite differently from what's considered the norm for that particular constriction, dependant on a considerable number of factors. The only way to obtain a meaningful determination is to pattern a specific load, with an averavge of at least of 5 shots, at 40 yards, enclosing the greatest density of the individual shot patterns within a 30" diameter circle. A variation of 5-10 percent or more is not unusual when brands of otherwise identical loads or even shot sizes are switched. And don't rely on the published number of pellets per ounce ... cut a sample load apart and actually count the pellets.
Time consuming and even boring ... but you will learn something ... whether or not you can actually put it to beneficial use is another matter altogether.
If your shotgun doesn't fit, i.e., "hit where you look", choke constrictions won't matter much.
Most beginning shotgunners would do much better, IMHO, if they concentrated more on developing sound shooting techniques, a consistant gun-mount, developing a smooth swing and follow through, and use a little less choke with good shells.
