http://nielsenspecialtyammo.com/pages/swaged-pellets-for-air-rifles

Swaged pellets are nicely formed as well as accurate ammo for high powered PCP air rifles and also pellet guns. These pellets, often referred to as bullets or slugs, are formed in a swaging device. Using cool lead, a core is positioned right into the die and pressed from top and bottom to develop it right into a pellet.