Bullet swaging is a method of using pressure to form a bullet, which is the inert metal portion of the cartridge (which becomes a projectile when fired) by using pressure to flow the materials at room temperature.
Unlike casting, no heat or molten metal is used. The high pressure is applied with a special, powerful press (either hand or hydraulic powered) designed to hold a precision, diamond-lapped die (the vessel that holds the pressure and controls the bullet diameter) and both an internal (inside the ram) punch and an external (in the press head) punch.
The punches press against the material inside the die from both ends, and cause it to flow, filling the die and taking on the exact shape and finish of the die and punch ends. Safety, speed, precision, versatility, and low cost for the number of weights are some of the benefits of swaging.