The illustrated firearm is a Merwin & Hulbert Army Revolver, made from 1876-1880. It is a late type with topstrap and fluted cylinder. The 5" barrel is uncommon, if it has not been sawn off, 7" was standard for the big guns.
The CALIBER is Winchester 1873, commonly known as .44-40.
They have an interesting and complicated action, gate loading but simultaneous selective extraction. That is you can unlatch and cam forward the barrel and cylinder, leaving shells held against the breechface by the rims. The empties will fall out, live rounds will be retained by the length of the bullet still protruding into the chamber.
Gunsmith David Chicoine recommends they not be shot. There are some thin areas of metal carrying the load and the steel was no better than you would expect for the time and maybe not as good. They are very precisely made and fitted though, just fun to play with and feel the smooth operation. If in good condition you can swing the barrel and cylinder forward as though to unload and feel the suction of the closely fitted axis pin.
Dollar value is probably pretty good, the peeling nickel plate hurts, of course, but the bone or ivory grips are a plus, as is the intact lanyard loop.
There is an outfit trying to rev up a reproduction, you can pre-order at a starting price of $1250, 20% deposit. I doubt they will ever be seen, but we can hope.