The equation is
. If that image doesn't work, here it is in programmer notation.
Terminal Velocity = Sqrt(( 2 * mass * g) / ( p * A * Cd )).
Cd is the hard one. With spin it will be constant, but with a tumbling bullet it will vary. A larger sphere of lead is going to have a much higher mass/area ratio than a small one. No one needs to worry about bird shot raining on them.
But heavier skydivers fall faster and must wear suits with higher drag coefficients to slow down, and light skydivers wear weights.