I appreciate that Ivan I learn something from you guys every day thank youthe parrel grooves are for the cartridge crimping tp seal bullet to case as a gas seal that means its a cartridge case type bullet the hollow back is typical of black powder type bullets to allow the gases to swell outwards making a good fit with the barrel rifling thus its post civil war era blackpowder cartridge ammo pre smokesless powder so about 1880 to early 1900 era