Shotgun Bore/Gauge Sizes


Maybe of use to some.

Note, I believe that Letter sizes were only used for punt guns, whereas Number sizes were used for punt guns, bank or rail guns and pure shoulder guns.

English Shotgun Bore Sizes.pdf (186.2 KB)

When I say bank or rail guns these were technically shoulder guns but were used rested on a bank side or a fence rail and were not used standing and swinging at the target although some were carriage mounted.
People referred to punt guns by the measured sizes i.e. 2 inch (2.000") or 1 1/2 inch (1.500") rather than the letter bore size.


Thanks…very informative information. Had never been 100 percent sure that bore size and gauge were based on the density of HARD lead, but now I am.
Amusing how some “sources” quote PURE lead, but then give examples and/or formulae based on hard.



This does not have letter sizes, and I do not remember where- or when- I acquired it, but it does have measurements of shotgun bore sizes/diameters.
Shotgun-Caliber Size Equivalents.pdf (53.9 KB)


Eightbore, thanks a lot for the information.


Could anyone give a quick explanation to why 36 Gauge is not the same as .410…?

Regards Finn


Can’t answer to the charts but in the US some United States Cartridge Co. brass & paper shells were headstamped .410 / 12mm


Hi Finn,

36 Gauge = .506" diameter based on 1/36 of a lb ball of lead
.410 = .410" diameter based on 1/67.62 of a lb ball of lead

assumes a density of 0.4097 lb/in3 for lead.



Thanks for the replies,gentlemen…
And sorry for not understanding this…(O:
It seems that Italy is calling them cal 36
England and USA .410…

As long as we agree that they both go in the same gun…


Morning Finn,

There should be 0.096" difference in the bore diameter of them, technically speaking they are NOT the same cartridge, if you are saying they are the same diameter (which they look in the picture) then you have me stumped.


EDIT; Just found this on the internet;
A shotgun “Bore” is measured in inches, and only a .410 shotgun uses this actual measurement. The “Gauge” of a .410 Bore is actually a 68 gauge (often referred to as a “36 Gauge” though no one is sure why as the maths don’t work).


Thanks,then i guess its not just me who has wondered about this…
The diameter is the same,and RC,Italy even mark them both cal 36 and 410.



12 mm, 410 and 36 gauge are exactly the same ctges
I cannot explain to you now the history of designation for each of these ctges because I am in Vietnam but I will do it as soon as I am back


Thanks JP,looking forward to it!
Always believed that 36 and 410 was the same cartridge,just got a little baffled when I saw Eightbores
list where they didnt match up… I dont think I have seen cal 12 headstamp.


Boxes, I don’t seem to have any with all three nomenclatures on them.


Hi Finn,

Need to be a bit careful here with just 36 as this is a .360

So it could lead to more confusion, need to be clear it is a 36 Gauge or a .360" Bore…


I only posted the table as I thought it would be useful, seems it has caused confusion instead.



Sharing historical data that is not readily available is an excellent idea. I am glad you did it. Historical data is prone to being contradictory.


Yes you are right there. Problem is that it is rarely the “Historical data that is prone to being controversial” but rather the way that it is interpreted and used and miss-used that creates the problems. Added to this we seldom know exactly how it was meant to be used, as the table states, bore / gauge sizes only go down to 50 and after that they are dimensional sizes.

People also seem to mix up the cartridge diameter with the barrel bore size and forget the chamber size, which is where the 12mm came from I believe (approximately the head size of a .410 cartridge). Gets even worse as a .360" cartridge measures approximately .410" !!, so bore size and cartridge diameter are easily mixed up. So where the 36 Gauge comes from I shall wait with baited breath for JP’s explanation, because I am not going into that minefield :)…


Finn, is “73” the length pf the FIRED casing?


Here is my small understanding, and explanation, as best I can:
In the U.S., the .410 originated with the .45 COLT shot cartridge, which was originaly crimped, then lengthened and roll crimped to the length of a loaded cartridge, when fired, to get more shot into the casing, hence a caliber designation, not a shotgun bore size. This was for the “Trick” shooters primarily.
Eventually it was lengthened and made into its’ own cartridge, and applied to shotguns, since it is a “shot” cartridge, yea?
One would need to go back and see the evolution of the bore diameter of the European cartridge to understand the difference in measurements, like the .44 Spl/Mag which are .429 because of the heel bullets first used when the cap and ball cylinder was bored through for a cartridge.
ANd the 7.62x54R which ain’t .308 daimeter, among a few dozen others.


Hi Jack!

Yes it is:

And thanks for your explanation!


the French increased bullet diameter from 8.1 mm to 8.3 mm when going from the original jacketed bullet for the Lebel rifle to the pointed balle D (massive gilding). Germans and Russians also increased their respective bullet diameters when switching from round nose to “spitzer”. This is the reason why 7.62 x 54 R uses a larger bullet diameter than .308".