Variable velocity variable trajectory piston propulsion ammunition case

A method of firing a medium caliber round of ammunition at variable velocities and trajectories, comprising:

• providing a medium caliber ammunition round (10) disposed within a firing chamber (14) of a rifle, the ammunition round (10) comprising a grenade (12), and a cartridge (20), the cartridge having a longitudinal axis (30), a front case (22) having a forwardly extending flange (23) and channels (44) through the front case walls parallel to the longitudinal axis (30) where the grenade (12) being seated within the flange, and a rear case (24) with a primer opening (28), a cylinder (42) in axial alignment with the cartridge (20) having a chamber therein and vents (43) about the cylinder's peripheral interior walls, the vents (43) extending radially outward through the cylinder walls and being in fluid communication with the channels (44), a hollow piston (40) in axial alignment with the cartridge (20) and slideably disposed within the cylinder (42) chamber and having a cavity (41) facing the primer opening (28), the piston (40) having a combustible compound (50) disposed within the cavity (41) for creating a combustion gas (52) upon ignition, the piston (40) further being movable between a first position in which the rear of the piston is adjacent the rear inner wall of the rear case (24) and the front of the piston sits flush with the cylinder (42) forward end and is further in contact with the rearmost portion of the grenade (12), and a second position where the piston (40) is extended forward beyond the cylinder (42) forward end and the cylinder vents (43) into the second chamber (45), such that the vents (43) are exposed within the cylinder (42), a variator collar (26) disposed on the cartridge between the front case (22) and rear case (24), the variator collar (26) being adjustable between a first position reducing fluid flow between the vents and channels and a second position increasing fluid flow between the vents and channels within the front case;
• adjusting the variator collar (26) to a position between the first position and the second position;
• striking the primer (28) within the rifle to ignite the combustion compound (50);
• igniting the combustible compound (50) within the piston (40) opening to create the combustion gas (52);
• propelling the piston (40) through the cylinder (42) chamber from the piston first position toward the second position to move the grenade (12) forward to form a chamber (45) forward of the front case;
• continuing to move the piston (40) forward to the second position to expose the vents (43) in the cylinder (42);
• causing the gas (52) to flow from the vents (43) within the cylinder (42) through the channels (44) to the chamber (45) formed forward of the front case via the gas flow volume, the rate of volume established by the position of the variator collar (26);
• imparting thrust to the grenade by the propelling piston and subsequent gas flow;
• and
• firing the grenade (12) out of the cartridge (20).