Complex sentence: Sights on the new pistol are different because of manufacturing changes; first-generation slides are milled from hex-bar 4140 steel while second-generation slides are milled from square-bar steel.
Split into simpler sentences: Sights on the new pistol are different because of manufacturing changes. First-generation slides are milled from hex-bar 4140 steel while second-generation slides are milled from square-bar steel.