Its a very nice gun, very nice.
I believe its possible to make a double rifle again from this with a liner. I have a Mauser C96 pistol, you call it Boomhandle, with a srew in 9mm Parabellum liner in the original 7,63mm Mauser barrel. the 577 snyder cartridge have a max. pressure of 1600 bar when the 9mm have allmost the double pressure. look for a good craftsman!!
It may be a problem to find a .577 barrel with the same land/ groove dia. as the old one in your gun.
The 577 Snyder cartridge was maybe available from 1865 or 1866. go to the British Militaria Forum, there are the Snyder rifle experts. I can imagine that this gun startet as a muzzle loader. Knowing guns, converted first time for the pinfire cartridge and some times later for the center fire cartridge. such things happen and when only the barrels come from the muzzle loader. you can find offers in german catalogs from 1914, before WW 1, that they have converted every old muzzle loader into breechloader for low money.