Abstract: Apparatus for providing a controlled increase in muzzle velocity of a projectile while reducing peak value of gas pressure inside a gun barrel. A cartridge includes an elongated body having a central bore divided into three chambers, with a fuel chamber separated from an oxidizer chamber and an elongated capillary chamber by a plurality of membranes. A fuse wire and a power supply vaporize a plasma base in the capillary chamber and provide a controlled jet to provide combustion between a second fuel in the fuel chamber and an oxidizer material in the oxidizer chamber. The power supply controls the fuel-oxidizer combustion rate to obtain a relatively steady pressure of long duration against the projectile which results in high projectile velocity with relatively low peak values of pressure in the gun barrel.