You left out terminal velocity.
I did the math at post 15 but you can check me.
Think of it as a vector with two components, vertical and horizontal. The vertical component by itself, that of a spent bullet falling at terminal velocity (which would not be the same as for a penny, since a bullet is not a penny), is not enough to do serious damage in most cases. If you add a horizontal component, though, the resultant is more than the vertical component alone.
Also, if the bullet has more mass, heavier, it would not go any faster coming down, but would impart greater energy to the object it strikes. Think brick and penny, both at terminal velocity, which is a function of the air resistance causing the falling object to "float" as the air resistance equals the gravitational acceleration.
Indeed, some projectiles will penetrate that skull with sufficient force to scramble the soft tissue inside.