Major General Julian Hatch, a U.S. Army firearms expert, did extensive testing on ballistics and falling projectiles in the 20’s. He calculated that .30 caliber rounds will reach terminal velocities of 300 feet per second (90 m/s) on descent, and determined that, while most bullets will leave a small dent in the ground when they land, that same bullet travelling between 200 to 330 feet per second can still penetrate human skin. The experience of many hospitalized and killed innocents validate his conclusions.The popular scientists on the TV show, Mythbusters experimented with the premise that “Bullets fired into the air maintain their lethal capability when they eventually fall back down.”
They found that a bullet fired straight up (an almost impossible achievement for a human), will tumble on its return trip and falls at a slower rate due to terminal velocity. In addition, they found that a bullet in this circumstance is therefore less lethal on impact. However, they also discovered that a bullet fired at a non-vertical angle will be able to maintain its speed enough to be very destructive on impact.
The non-vertical angle has to be significant for the retained velocity to be significant.
At about 40 degrees, the velocity at impact is a bit less than 600 fps for military, streamlined rifle bullets.
Everything else would be less than that, because they would have to travel through more air and lose more energy.