Abstract: A ram cannon (1) has an extendable barrel (2) slidably disposed over a starter cannon barrel (7). Such an extendable barrel is fully retractable during transport of the cannon and extendable at the time of firing. The barrel is also partially extendable, thereby providing a means to vary the muzzle velocity of a projectile (6) without varying the ratio of the fuel to the oxidizer in the propellant mixture. Utilizing an extendable barrel allows adapting a ram cannon to mobile artillery weapons, providing ease in transport, setup and firing, with a precise and rapid range adjustment capability.