I used to know a guy (who lived in Ireland) who had converted a 25mm marine flare pistol into a single shot 12ga pistol.
A local bartender used to keep an antique Remington 12 ga flare pistol under the bar. It was one of the brass framed Naval models from the 19th century.
I saw one just like it at the antique store years ago, and wish I'd bought it at the time.
Not sure just how much pressure one of those guns would handle but it looked like sturdy frame.
Inserts with threaded breech caps can withstand the chamber pressure, The flare gun barrel just acts to hold the insert in place.
Some time ago at a gunshow I saw a German double barrel flare gun pistol, I think these were U-Boat issue.
There was a similar but much smaller double barreled teargas pistol, it looked to be made of zinc alloy, that one might have held up to garden gun 9mm shotshells but not much else.
I have an antique Remington line launching gun action (belonging to a friend's wife)which someone had roughly converted to a shotgun. I ditched the gaspipe barrel and I've been keeping my eye out for a proper replacement barrel.
These line lauchers were built on ten or twelve gauge shotgun frames but chambered for a .45/70 blank cartridge.
We'd figured on rebarreling the old gun as a flare launcher for their sloop but they just sold the sloop.
Not sure what we'll do with it.
Breech face needs to be bushed before I'd try any restorations.