How do you convert gauge to mm?

Answer 1

Assuming this is a firearm question, and you mean gauge of a shotgun barrel, and not wire gauge-

There is no direct conversion formula, since gauge refers to the number of round lead balls that fit the bore that would equal a pound in weight- 12 balls that fit a 12 g barrel weigh 1 pound, 16 balls that fit a 16 g weigh a pound, etc. The SMALLER the number, the larger the diameter. However, there ARE industry standard measurements for different gauges- you can look up the standard measurements, or contact me through my message board if you are looking for specific gauge.

PS- a .410 shotgun is not a gauge, but an inch measurement.