How big in inches or mm are shotgun guages?

Question

How big in inches or mm are shotgun guages?

Answer 1

4 bore .935"

8        .835"

10      .775"

12      .729"

14      .693"

16      .662"

20      .615"

24      .580"

28      .550"

32      .501"

410    .410"

And you may run across a 9mm shotshell, self-explanatory.

Let me put in a plug here for the 28 bore, which is a better alternative than 410 and easy to forget. Unlike the other odd guages, it's still in production and not going away.

Answer 2

The .410 is 41/100 of an inch diameter in a straight or open bore. There is also an obsolete .360 bore shotgun (36/100).  The remainder of the shotguns were numbered from the number of lead round balls that it would  take to weigh one pound. The ball would just fit in the barrel of the gun.  This is when the barrel was "open" or "cylinder" bore.  Hence a 16 guage would have a round ball that weighed one ounce and 16 would make a pound, a 12 ga would have a larger diameter barrel, hence each ball is heavier and then the number to make a pound is less.  I realize this is not the exact answer you wanted, but a 12 is about 3/4".

Answer 3

12 Gauge =  18.53mm  or  .729 caliber

20 Gauge =  15.63mm or .615  caliber

.410 caliber (inches) = 67.5 Gauge or 10.41mm