These weapons can vary greatly, but the common feature is that every pull of the trigger releases one bullet, and loading a new round is automatic. These guns are sometimes called “automatic” weapons, but in this case, that term refers to the loading, not the firing.
The automatic loading process usually involves recycling some of the gun’s gunpowder gases or momentum and using it to eject the empty cartridge and load a new one. Many semi-automatic rifles have external magazines holding five to thirty rounds, which can be changed quickly to reload the weapon. Examples of semi-automatics include the AR-15 and Browning BAR rifles. (Notice the difference in the appearance of both.)