Far as I know...
Cap and Ball Revolver, or, very old Metallic Cartridge Revolvers...Pure Lead was the material suited for their Bullets, whatever shape those Bullets may be.
Pure Lead will slump or upset enough when accelerated in BP Revolver loadings, sealing off Gas leaks around it, and, widening itself into the Rifling.
Copper Jacket Bullets will not slump or upset enough at the rate of acceleration which is probable in a BP era Revolver, and, in a Cap and Ball Revolver, will have to to fit into the front of the Cylinder to load, making for a poor Barrel-Rifling Fit once they do not upset, or, if in a Metallic Cartridge BP era revolver, if they are the right size at all, having to be squeezed down by the Forcing Cone, then, and being a lot harder to squeeze, will delay the Bullet's acceleration slightly, while pressure, or, more importantly maybe, duration of rising pressure, continues behind it, stressing the Cylinder and Forcing cone area.
Legends abound of people melting down Pewter or Britannia Metal in a pinch, for casting Balls for Rifles...this does not mean it worked well, or did not suffer accuracy issues, even if patched, as they would have been for use in Muzzle Loading Long Arms.
The Metal would be too hard to slump or upset very well to press itself and or it's surrounding Patch into the Rifling...and would be a lot worse in a Revolver or Single Shot Muzzle Loading Pistol.
In a Cap and Ball Revolver, the results would be terrible, if the Balls even stayed in the Cylinders on their own at all.
Of course, you can shoot Gravel, old Screws, Birdshot, Nuts and Bolts, Carpet Tacks, small chunks of broken Glass, stacks of Washers, or anything else, over a thich enough wad and top-wad for that matter.
Plain, pure, soft Lead...is what these Arms were meant to use...whatever the Bullet shape, and, is what will work best in them.