Howdah Pistol - Double-Barrel Pin-Fire

[Patrick Chadwick]

Looking at the breech photo in post 15 (thanks UNPROFOR1994), it looks as if it was brutally reamed out with a builder's drill! So we must reckon that the original chambering was a bit smaller.
Looking at the rim of the ACP case in post 20 and using it as a reference to scale up the recess in the breech, the original case had a rim with a MAXIMUM 14mm (0.55") diameter. Something around 13mm +/- ?

Now to look for some plausible cases...

One candidate is the German pistol cartridge for the "Reichsrevolver". Designation (there are several alternatives) 10.6x25R. With a bullet diameter of 10.92mm (nominal).