They switched gradually over about a 100 year time period. Starting with muzzle loading weapons going into the first breech loading weapons in the early to mid 1800's (Hall's Rifle is one of the first American breech loaders.) Then going from breech loading rifles into lever action rifles in the 1860's. Many of these developments overlap each other as well, so there is really no set date where everything changed, but it was a gradual change over the 1800's, ending with stripper clip fed bolt action rifles. The first system you mentioned can fall under a few types. Muzzle loading weapons are loaded from the muzzle. Breech loading rifles are loaded from the breech, which is covered by a moving breech block, or sometimes the breech itself moves in and out of the frame. Also, some lever action weapons also were loaded one bullet at a time, but held more than one bullet in the internal magazine. I hope this is clear..