replying to rdelaney2001, Iggy wrote: Time is the main calculation factor. The bullet leaves the ground at top speed, but is constantly slowing to a literal stop. So, its traveled distance is extremely shortened due to its dropping speed allowing the effects of gravity and air to greatly increase.
Now, it starts back down from zero, instead of an accelerated speed. Let's say the bullet left the gun at 250 m/s and took 5-seconds to hit its ceiling. If it also took 5-seconds to fall back, it would only be going 50 m/s (generous, maximum) at gravity's 10 m/s/s. At 10-seconds it would be 150 m/s. Only if it took 15-seconds or more would it then have any chance of coming back at the gun's 250 m/s.
In reality however, proven by trial and error and not at all by physics or math, then more factors come into play. The biggest being Terminal Velocity, where the bullet itself acts as its own speed limiting parachute...similar to the bullet's immediate slowing as soon as it left the gun, but longer lasting.
The facts are that bullets can still harm and even kill, but in all cases they're never going anywhere close to their fired speed. Roofs have been hit with only penetration of the roofing material itself, but not even the thinnest roof structure. People have been hit and the bullet bounced off a bone under the skin. Car glass has been hit, but not penetrated or breached and only shattered or bulls-eyed.