A medium caliber round of ammunition capable of being fired at variable velocities and trajectories, and methods of firing such, are presented herein. The ammunition round includes a grenade and a cartridge received in a firing chamber. The cartridge has a front case for receiving the grenade, and a rear case for receiving a primer. An adjustable variator collar is disposed between the front and rear cases. The variator collar is adjustable between a first position that chokes the venting of gas from a high-side stage to a low-side stage for low muzzle velocity, and a second position for un-choking the venting of gas from the high-side stage to the low-side stage for a rapid rise in pressure during the low-side stage, resulting in a high muzzle velocity. The collar may further be adjusted in any position between the first and second for relative amounts of controlled fluid flow.