An experimental device that additional chambers were mounted on a barrel of 14.5 mm ballistic machine gun was adopted in order to study the pressure platform-like effect of increasing muzzle velocity with a barrel additional charge. The 6/7-XDGZB gun propellant was used as the main charge, and the 4/1-XDZJ was used as the additional charge. The results of closed bomb experiment and interior ballistic experiment show that the 6/7-XDGZB gun propellant has the characteristics of lower initial burning rate, and the 4/1-XDZJ gun perpellant with higher burning rate has property of regressive burning. The two charges burn in coordination in their respective chambers, and the muzzle velocity of the additional charge can rise from 981 m·s^-1 to 1063 m·s^-1 compared to the blank charge (i.e. main charge only, no additional charge) under the condition in keeping the maximum pressure unchanged, and piezometric efficiency can rise from 0.47 to 0.58.