In an open chamber air gun, a round is gravity fed or magnetically fed into the open chamber and falls into the magnetic flux lines of a magnet disposed at the chamber. Due to the magnet, the round is pulled securely into the chamber and then remains momentarily held in place, blocking much of the air behind it and thus increasing the air pressure driving it. When the round moves down the barrel from the breech toward the muzzle, it quickly leaves the vicinity of the magnet and the magnetic drag ceases, after which the air pressure is free to drive the round to full velocity. High rates of fire and muzzle velocities are achieved with moderate air pressure and an action with no moving parts.