[Charles H. (The_r0nin)]

Actually, the cross-section of the projectile will give a pretty good maximum case (most bullets will actually have less air resistance than that due to shape and spin). I'd settle for the maximum air resistance, because I'd still be willing to bet it'd clear a mile-and-a-half...

[mamelukesabre]

football shaped projectiles will tumble when they fall below a certain velocity. Tumbling increases drag by an order of magnitude at least.