The Four-ten is also known as the 36 gauge ( and 12 mm ).
A simple calculation of the number of lead balls of a certain diameter that can be made from one pound of lead (the usual calculation of shotgun bore sizes - See shotgun bore sizes) shows us that about 68 balls of 0.410" diameter can be made from 1 pound of lead, hence a four-ten should be a 68 bore/gauge!
The above diagram of (approximate) cartridge sizes also indicates that 12 mm is also not too accurate ( though not of far out as 36 gauge is from 68 gauge)!

We must thank Michael Goines for raising this question of the mythical 36 gauge and the original research that he has performed in trying to get an answer.
The following is an Email containing some very interesting research (Pietro is not an 'Historian' or 'Expert', just an Amateur with a keen interest. Much of the enclosed information is from recollections from very knowledgeable, though old, persons and there may be little or no documentary evidence to back it up.)
We thank Pietro for permission to publish his research, which must have taken much time and effort. ( if any one would like to contribute any more infomation on this topic please email ..

Dear Michael:

your question intrigued me and i started researching on it.
I knew that 36gauge was an "artificial" denomination of a .410 bore caliber, but i wanted to know when, why and who did it...

the task was bigger than i thought...

nobody to this date has come up with a very precise answer, even the president of CIP (the european ruling committee on arms and ammunition).
This is what i found out is the following:

1 - all the official documents from european Proof houses before 1904 do not mention .410bore caliber.

2 - In Great Britain, in a 1855 and some previous documents, official gauges went from 1 (1.669") to 50gauge (.453").
In a later (1868) document, they increased the list to go from A gauge (2.000") to 50 gauge.
In all documents, 36gauge reported a .506" diameter.
The gauges were determined with the number of lead balls of that diameter with a British pound.

3 - France, in 1810, try to get away from the british system and they managed to keep alive two systems: one was similar to the British (except the french pound was different) and determined gauges fairly similar in diameter to the british system;
the other, called the bore system, was similar, but used the kilogram (for example a 32 bore was very similar to a 12 gauge).
In 1868, they killed the bore system and tried to rationalize the dimensions. They still based the determination of the gauge on the number of lead balls made with a french pound, but they decided to adjust the diameters to have 0.2mm steps between gauges.
This is probably were the .410 was born (even though was not called so; officially it was called a 12mm):
in fact, the french proof house decided that all the guns smaller than 10.6mm (roughly .410") had to be tested for pressure in a different way than the bigger ones. So, .410 became the divider between serious guns and play things.

4 - In Germany in the 1800's there is no mention of any gauge smaller than 32 (and by the way they used several different "german" pounds, depending where the gun was manufactured)

5 Austria had a system similar to the english, from 4 to 50 gauge. There was a 36 gauge with diameter 12.4mm (surprise: it is different from the french and english 32...)

6 - Italy was a mess: depending on who was the invader (Austria or French or Spain) they changed system.
The presence of more than 30 weight systems in the territory, complicated enormously the situation. Basically, in the 1800's there was no two guns alike in the entire european continent...
luckily the european gunsmiths were pretty good in making custom made balls after measuring the gun barrel. Things started to change in the 1900's, probably because of the need of having standard arms and ammunitions when assembling armies of different countries.
Here we go again:

1 - the first official reference to .410 bore caliber is in a 1904 document by the Royal british proof house; the same document has a 36gauge (with the "correct" .506 in diameter).

2 - CIP met for the first time in 1914 and managed to get an agreement on the nominal diameter of calibers from 12 to 28gauge (12, 14, 16, 20, 24 and 28). There was still some resistance on 4 and 8 gauge and other bigger calibers (up to 32 mm, which was an italian 1 gauge), and french and british 8 gauge and 4 gauge stayed until the 40's, along with the official european 4 and 8 gauge. In the 20's and 30's 14 gauge disappeared and 32 re-appeared.
All the other smaller calibers (with the exception of .410 bore) disappeared completely.

3 - sometimes in the 20's, someone at CIP (mistery, probably a swiss or a german..) probably thought of making an ordered and esthetically pleasant set up...since they had 4, 8, 12, 16, 20, 24, 28 and 32, why not calling the next smaller (and only remaining) caliber 36 (a precise 4 step).
Later they reversed to using the correct .410, but the industry had already started using the two names.
There are some 1920's catalogs from Fiocchi and Dynamit Nobel using both 36 and .410 for the same shell.

4 - In the 1961, CIP officialized .410 as the only correct name, but in 1969 added 36 in parenthesis on the dimensional tables.
Basically, they were acknowledging the situation.

5 - The confusion never died, because the french kept calling the 32 gauge 14mm, the .410bore 12mm and they added the .360, calling it 9mm (later to become a rimfire, with the name of Flobert...awesome story too).
In Italy and other european countries used 36 gauge for the shorter .410 (2 and 2 1/2" long) and .410 for the 3" long, also called 36 Magnum.
I still don't know exactly who and when created this, but i am 99% sure there is no real technical explanation behind it and it is the result of trying to get an agreement between several countries and several hundreds arms and ammunition producers, all of them with their history and reasons.
The fact is that 36 gauge and .410 bore now refer to the same shell. If i get a better answer i will let you know.

I have to thank you that you gave me an excuse to get away from the normal day-to-day routine.
By the way, some of the best sources on this kind of stuff are from a Chicago company: The Gun Digest company.

You can still find some of their books in the out-of-print sections of internet booksellers.

My best to you and to all your shooting buddies,

Sincerely, Pietro Fiocchi