a selectable cartridge is provided consisting of a cartridge head , a dual - bed propellant chamber , a primer , and a tube . the invention enables the user to select a launch velocity prior to chambering the round . a high - velocity projectile is launched by mechanically rotating the base cap and rotation band such that holes along the primer align with the larger of two propellant beds . a low - velocity projectile is launched by rotating the base cap and rotating band a quarter turn from the high - velocity setting such that the holes along the primer align now align with the smaller propellant bed . the invention is suited for use in shotguns .