Look at the pictures in post #15 by dttuner.
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@dttuner: I was wrong about the gun maker Albert Simonis, I thought your gun had a spangled AS marking but it's a spangled AC marking.
That is not the marking of Simonis but a post 1877 controllers countermark of the Liëge proof house.
Hey guys, I found a link to Alan Daubresse's website. This guy is a published expert on Belgian firearms and proof marks.
Just sent him an email, so let's see what he says!
Website is very simple, and oldschool, but no malware for me.
Photos d'armes de collection
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).
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.
No need to argue over this. The proofs look genuine-enough Belgian proofs for me, and I have seen several pistols like this on the market here in Germany. As already pointed out - nothing to do with tigers, just a cheaper alternative to a real revolver for situations where one shot is not enough.
And the best candidate I can find for the chamber is....
... the 11.3x36R Gasser-Montenegro. A very hefty pistol cartridge indeed - 44 to 47mm OAL depending on the bullet and case version - longer than a 45 Colt, and not something which you would like to use for a target shooting competition. But possibly the best "man-stopper" of its vintage. An awful lot of "oomph" in that case! After repelling the assailant you probably needed a visit to the physio to have your wrist treated.
Anything longer than the Gasser would seem to be a carbine cartridge*, and hardly believable for a handgun - although I once saw a revolver chambered for the Snider cartridge!
However, the Gasser case has a rim diameter of about 14.2mm. Dttuner, could you please use your calipers to measure the INSIDE diameter of the rim recess, to see if a 14.2mm rim would fit?
*In fact, the long Gasser has similar dimensions to the cartridge for the M1867 Werndl carbine. Surely enough for anybody's wrist?**
** In 1882 it was replaced by a shorter version, and the remaining army stocks were sold to Montenegro. Hence the commonly used name.
"And at some point one of the previous owners had the chamber reamed out so that the gun could shoot .410 shotgun cartidges."
Also a very plausible possibility. In fact, considering the extraordinary chamber length, the most likely.