If the previous post is correct and that is a stage gun, or a starter pistol, it is not designed to fire actual live ammunition and will explode in your hand causing you considerable, if not fatal, injury. Guns of that type fire blanks and lack the hardened barrels needed to withstand the pressure of live ammo.
Do not continue to try and force a cartridge into the barrel as it may accidently discharge as it appears that the slide is contacting the primer in one of your photos.
"The whole of the Bill (of Rights) is a declaration of the right of the people at large or considered as individuals.... It establishes some rights of the individual as unalienable and which consequently, no majority has a right to deprive them of." (Albert Gallatin of the New York Historical Society, October 7, 1789)
"A free people ought not only to be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government." - George Washington