Handloader's Manual of Cartridge Conversions shows .44 Ballard Long as 1.08" case length, 0.506" rim diameter, 0.052" rim thickness, 0.441" head diameter, and 0.440" neck diameter. The .44 Ballard Extra Long is the same, except 1.63" case length. Both show a 0.439" bullet diameter.
I have an antique Belgian 6 shot centre-fire revolver, made in an unspecified caliber during the transition from pin- to centre-fire.
It's chambers taper from 0.485" to 0.465", the cylinder is 1.328" long, and it's bore slugs to 0.427"
I initially tried shooting it with cut down .44 Magnum cases, using .44 Russian specifications, but the case-heads expanded way too much.
I now shoot it using .45 Long Colt cases shortened to 0.98", with the front 0.4" of the case sized down in a 44 sizer die to provide neck tension for the 0.429" lead bullet over 20gr. FFFg black powder. Kind of like a squatty 44-40, but it works.
There are several other US .44 handgun cartridges listed in HMofCC, and who knows what the various European manufacturers were making.
If you can, do a chamber cast, slug the bore, and go from there. You should be able to get it up and running with a bit of experimentation.
Regards, DaveF.