I don't know what it is

It is a fired rimmed cartridge, 10.5mm ID at the mouth, about 26mm overall length, rim about 13mm, 11.5mm above the rim, what is it?

The 10.4 x 26 R Swiss Short Revolver is close to the dimensions you have given.

I think you mean 10.4x20R Swiss, but mine is longer. There is such a thing as 10.5x25 R Mauser but my headstamp base is totally flat.

I took the info from the ECRA data disk, it shows:

10,4 mm Swiss Regulation CF M.1878

Year 1872
Country Switzerland
Weapon Revolver
Projectile Round nose, paperpatched lead || Round nose, FMJ (tombac)

Specimen # 6 Measured specimen

Projectile Round nose, FMJ (tombac)
Headst.

Bullet

Thanks, would you happen to know which of these revolvers fired this calibre?
swissrifles.com/pistols/index.html

Sorry can’t help.

As a pure guess could there have been a civilian version of the M1878 which took the 10,4mm x 26 R Swiss Short case ?

Whats the betting it’s actually a .44 Russian ;)

Dear Armourer,
I need help, when I enter “10.4 x 26 R Swiss Short Revolver” into general search, nothing shows up. May you provide me with a link?

[quote=“sksvlad”]Dear Armourer,
I need help, when I enter “10.4 x 26 R Swiss Short Revolver” into general search, nothing shows up. May you provide me with a link?[/quote]

The only reference to the 10.4 x 26 R Swiss Short Revolver round I have is on the ECRA Data disk.