Guns fired
exactly straight up.
Pistols & shotguns (with shot, not slugs) are unlikely to be lethal. Shot doesn't have the sectional density to have a really fast terminal velocity and pistol bullets (in the only known experiment where any were recovered after being fired straight up) tumble which dramatically increases the drag (and therefore dramatically decreases drag) on the "return trip".
Centerfire rifles have the potential to be deadly since the bullets (in the only known experiment where any were recovered) remain spin stabilized and fall base first at 300-400fps. That's not certain death, but with a little bad luck it can do the trick. A shotgun slug is big and heavy enough that I would expect it has the potential for real mayhem even at pretty low velocities--primarily as an impact weapon rather than a penetrating projectile.
Shooting at any significant angle from vertical (up but not exactly
straight up) is very dangerous for two reasons.
- The bullet typically remains stabilized and traveling nose forward which makes it much more aerodynamic. That means much higher terminal velocity.
- The bullet comes down with a velocity that is the vector sum of both the terminal velocity and any remaining horizontal velocity not bled off by air friction.
These will definitely have enough "oomph" to be lethal and it's not that hard to find evidence to support that fact. I did an internet search looking for deaths from descending bullets some time ago and found enough documented instances to convince even the most skeptical. I don't know if that was posted here or on THR, but by now most of the links are probably dead anyway.
At any rate, I found deaths, injuries and even some descriptions of roof damage. There is also a fairly high-profile law in the U.S. against firing in the air that is named after a young girl killed by a descending bullet.
As to the original question: I have seen a case where a handgun bullet from a drive-by shooting (probably a .357Mag SWC) traveled through a typical woodframe house. In the front, out the back and through everything in between. It happened to take a path that didn't put a lot of intervening walls in the way, but a handgun bullet can definitely penetrate two exterior walls and some more besides. Bullets are good at going through things, that's what makes guns useful.
Offhand I don't know of any cases where a bullet left one house, entered another and accidentally killed someone in the second house but that would only speak to the likelihood of such an occurrence, not of the possibility which is fairly well established.