The invention relates to systems, methods and devices for controlling the dispersion of shot or other projectiles fired from a weapon such as a shotgun. Embodiments according to the invention employ an inner cylindrical chamber that is divided at a median transverse plane into forward and rear sections. The rear section of the chamber tapers toward the median transverse plane and includes a number of radial vents that open into an outer chamber that provides one or more gas bypass channels into the forward section of the tubular chamber. The forward section tapers toward a forward muzzle opening. When a multiple projectile load/wad is fired from the weapon, expanding gas is sealed behind the load until it has advanced past the radial vents at which point, gas is allowed to expand into the outer chamber resulting in a momentary pressure drop and reduced propulsion force behind the wad/load. The relatively light wad decelerates while the much heavier shot continues forward, rapidly separating from the wad before the wad exits the muzzle. The wad seals against the tapered bore of the forward section of the inner chamber blocking the escape of gas from the outer chamber passageways until sufficient pressure develops to eject the wad from the muzzle end.