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

Swaged pellets are nicely formed as well as accurate ammunition for high powered PCP air rifles and pellet guns. These pellets, typically referred to as bullets or slugs, are formed in a swaging device. Utilizing cold lead, a core is placed into the die and pressed from top as well as the lower to form it into a pellet.