This here. I'm guessing it wasn't actually the acceleration added by gravity, but the energy remaining from the horizontal vector of the bullet's trajectory. Unfortunately, this is just a guess on my part. Hatcher's work for the .30-06 is of limited use, as bullet shape is going to influence terminal velocity. If this is the case... it is actually safer to shoot straight into the air than just kinda mostly into the air! If we did this, celebratory gunfire might actually be safer than firecrackers!