Did you account for air drag, or did you only consider gravitational acceleration?
I took an average pebble out my garden. It weighs 10 grams and has a mean diameter of about 20 mm. If we assume it were perfectly spherical with a drag coefficient of 0.47, travelling through air with standard density of 1.225 kg/m^3, and plug the figures into the formula for terminal velocity we get: sqrt((2*0.01*9.81)/(1.225*(3.14*0.01^2)*0.47)) = 33 m/s or 119 km/h. If we pipe that into E = 1/2*m*v^2 for energy, we get ~5.5 J or ~4 ft-lb.
Or in other words, I need Bitcoin to do something exciting.
Perfectly spherical bullets are way different to regular bullets that end up tumbling as soon as they lose the rotational speed with a way higher air drag.
Much of the penetration of a regular gunshot is thanks to its rotational speed. That's why pipeguns are not that effective, even at very short distance, unless properly directed against unprotected vital points (neck, eyes, temple, etc).
To the mooooooon!!! weeeeeeeeee
