Listen to DallasCowboy. BP only. The Winchester 1873 uses a weak toggle link action. Yours is considered an early 1873 being made in 1885 and the metallurgy wasn't quite up to snuff at that time for smokeless ammo. Buffalo Arms has black powder 44-40 ammo in normal times in stock at a reasonable price. Hopefully it will be back in stock sometime this year.I was told you could shoot modern loads through the Winchester as long as they weren’t high pressure or copper plated? I don’t know much about these old firearms so any advice would be greatly appreciated as they have been in the family a long time and I don’t want to mess anything up. I want to pass them along to my son’s some day.