The short version is the ratio between air resistance and mass. That ratio determines something’s terminal velocity. Pennies are not as dense as bullets and they are also flat. Hence they have a high surface area for their size/weight. So they fall slower than bullets. Some people claim bullets don’t kill coming back down but that is clearly false as people die to errant vertically fired bullets all the time.
most bullets fired errantly are not truly vertical they all have an angle to the horizontal that angle is absolutely critical to the. bullet keeping its gyroscopic spin stabilization and therefore it’s velocity higher than it’s terminal falling velocity, which as mythbusters measured for 9mm rounds in still air well under the speed of sound, and under what would be necessary to penetrate the human skull, bullets fired at even a small angle to the horizontal maintain their spin and never reach zero velocity at apex there is always a forward velocity vector in the parabolic arc they have to follow. that forward velocity and the fact that they stay nose first and spin stabilized makes bullets in an arc deadly while truly vertical bullets are not. I personally have fired a 9mm straight up while on open water and when it hit about 50 feet away the spash was tiny. there was a good breeze that day we knew there was zero change a vertically fired bullet can return to the firing spot with a horizontal crosswind physics dictates we were perfectly safe.
Ratio of air resistance to mass is not relevant, short version or long. Mass actually factors out of the equations of motion, and all that really matters is the acceleration of gravity and the drag coefficient per unit of mass.
All of the equations of motion for objects falling through fluids (like air) involve fudge factors, and those fudge factors are usually quite complex, have to be measured, and are valid only over a range of velocities.
Newton himself used an equation of motion derived from the Third Law which had a drag term proportional to a drag coefficient and velocity. That equation works pretty well if the fluid is not too dense and the velocity is not too great.
An improvement that's still an approximation uses a term proportional to the square of the velocity. In that formulation, the drag term is: ½ρv2AeffCD. Aeff is the effective cross-sectional area, and CD is the drag coefficient. [This equation becomes wildly inaccurate if the body tumbles, because then CD and Aeff become highly complex functions of time.]
Please note though, that for "stable" falling bodies for which the equation applies, since ρ the density, is m/V, the mass term appears in both the accelerating and the drag force and can be divided out: mg-½(m/V)v2AeffCD = m dv/dt. So you are correct in involving density in the discussion, but mass, per se is not part of it. You basically have to solve a differential equation which does not involve the mass of the body at all.
Well, technically, they're not fired directly vertical, but more often at an angle.
IIRC, a shot fired straight up, with no angle of deviation, will not likely kill you coming back down due to terminal velocity, because basically, once the energy going up dies, it's only subject to gravity coming back down
However, a bullet fired at an angle, even though it's fired "up", can kill because it would have gravity and propulsion working into the mix when coming back down.