At least to me...
 
Gettting tired of trying to find what the "caliber" was for a given shotgun gauge, I decided to put a table of such in with my other ballistic calculation spreadsheets. There is a nice formula for converting gauge to caliber (in inches), and having done it for the "normal" gauges - 10 to 32 - I decided to 'run it out' in both directions. So I did - from 1 ga to 911 ga. Why 911 you ask? 'Cause that's where the caliber is .172", the smallest commercially available projecitles. Hmm...
 
I decided to make sure the calculations were consistent with what tradition says "gauge" meant, which was the number of balls of the specific diameter in a pound. In expample, there are 16 pure lead balls of 0.662" diameter a pound. There are 12 such balls of 12 ga diameter, etc. Fortunately for me, the numbers came out right.
 
Then, after looking at the "small" gauges,I decided to make a table that had the number of pellets of a given "gauge" or caliber per ounce. This for determining about how many pellets of a given "gauge" would be in a 1 oz. charge for a shotgun. Since the diameters of the common pellet sizes are known, I could once again check my calculations against the know numbers. Again, miracle of miracles, the numbers "checked out". Then I noticed something...
 
Mathematically speaking, integers (whole numbers like 1, 2, 3...) are "interesting" when they show up among a bunch of "real numbers". ("Real numbers", mathematically speaking are those numbers with decimal points like 1.2, or 5.7, or 15.346...) When I looked at the caliber where an integer number of pellets/ounce occurred, I saw an interesting pattern. Have a look at the attached image.
 
In a nutshell, where there occurred an integer number of pellets per ounce, the caliber associated with that "gauge" was, for the most part, a common caliber. I'm always looking for an explanation as to why some "standard" was established. In the case of firearms history, there are precious few answers to "why" questions. Here may be some brief glimpse through the haze of history that had something to do why someone chose for example, .258 as a caliber.
 
Paul

A bridge to far for this lost sole. I just know that 12 gauge bore size is 1/12th of a pound of lead melted in a ball........and so on except for 410 which is a caliber. Regards, Rick.

So now I'm Jamie224ga?

WELL TYPO ALERT
 
Move the decimal point on 4 guage to .25 pellets per oz. Pretty interesing run when you look at it that way.

QuoteSo now I'm Jamie224ga?

Only if ya wanna be... :)
 
.410 is about 67 1/2 gauge.
 
For future reference, if you want the bore diameter of a particular shotgun gauge, the following formula will get you within a thosandths of nominal:
 
Bore = 42.416 * the cube root of (1/gauge)
 
If your caculator won't do cube roots, but will raise "x" to a "power", the equation becomes:
 
Bore = 42.416 * (1/gauge) raised to the 0.33333 power.
 

QuoteMove the decimal point on 4 guage to .25 pellets per oz.

Right you are Al.
 
Paul