Howdah Pistol - Double-Barrel Pin-Fire

[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.

I think it was made for the European market and the original caliber was one off the popular European 11 mm's at the time like 11 mm French, 11 mm German, .44 S&W Russian or .44 webley.
And at some point one of the previous owners had the chamber reamed out so that the gun could shoot .410 shotgun cartidges.