Abstract: A gas gun includes a chamber, a barrel, and a bullet charging member for introducing bullets into the chamber. An elastic O-ring is positioned between the barrel and the charging member. A stepped part is formed integrally with the chamber and permits radial expansion of the O-ring. When pressurized gas is introduced into the chamber, the O-ring is caused to advance to the stepped part of the chamber when urged by a bullet. The bullet is motivated by the pressurized gas. The O-ring thereby becomes radially enlarged such that the bullet passes through the O-ring and is expelled from the barrel.