The present invention relates to weapon systems that accelerate projectiles using gases generated by the rapid combustion of a solid propellant, in particular, such a weapon system is able to vary the barrel exiting velocity of the projectile through a barrel venting means. In one embodiment, a front venting means exhausts gas generated by combusting propellant from behind the accelerating projectile and redirects a portion of the exhausted gas either to at least one fixed volume, to the front of the projectile, or to a combination of at least one fixed volume and to the front of the projectile. Redirecting some of the exhausted gas to the front of the projectile restrains the projectile, thereby slowing the projectile, and thus further decreasing the muzzle velocity of the projectile. In another embodiment, gas from behind the projectile is exhausted into a fixed volume, thereby decreasing projectile acceleration, and thus, the muzzle velocity of the projectile. One can use a combination of fixed volume venting and front venting. A saboted projectile can be used for ammunition. By coupling the energy requirements needed to release the sabot to the barrel exiting velocity of the projectile, one can achieve a selectively lethal projectile. The venting means can be variable as to the velocity and mass of propellant gases exhausted or redirected and can be coupled to an operator selection switch, as well as to an automatic range finding scope.