The maximum estimated distance that anyone in this forum can throw a lead glans with a sling is 736 meters, based on SlingSim, a ballistics simulator authored by Matthias Borstad. The simulator estimates the effect of the shape of the projectile on drag, assuming that it is an ellipsoid. I ran the simulator on a 45 gram lead ellipsoid with a 3:1 length to diameter aspect ratio. The following are some results from the simulator, based on exit velocities that IronGoober has reported.^1,2

m/s	mph	Angle	Meters
78	174.5	41	530
82	183.4	43	575
95	212.5	43	736