It the hardness of the material used. Wheel weight even being harder than pure lead is not going to grip the rifling as well as copper will. So there is a limited slip factor envolved. But a casted hard lead bullet made from wheel weight should out preform most pure lead bullets. Has never played with bronze so I have no comparison vs copper jacketed bullets. In pistol shooting past 25yds I've found copper to be the most reliable and consistant.