A high capacity handgun/pistol is disclosed with a horizontally oriented magazine that is aligned with and disposed beneath the gun barrel. The magazine defines a plurality of hollow tubes arranged radially around a central axis about which the magazine can rotate automatically during operation of the handgun/pistol. The hollow tubes each contain a plurality of ammunition rounds, and can each be aligned with a loading tray one at a time, wherein a coil spring will push out a single round at a time onto the loading tray. The handgun uses a gun slide to load a new round into the chamber via the loading tray, and further enabling the loading tray to automatically rotate the magazine as each hollow tube becomes empty. Moreover, the slide will re-set a hammer to re-engage with a sear, after a pulled trigger releases the hammer to strike a firing pin. The handgun is further equipped with a selector switch enabling automatic and semi-automatic firing modes, wherein the semi-automatic firing mode can be further specified to select a three round burst fire mode. The handgun can also be configured with a simpler firing mode cam and fewer parts that limits it to semi-automatic firing mode. Additionally, the handgun barrel is separable and replaceable if the barrel becomes too hot after rapid fire.