So many reasons why not. For one thing, bullets get fired WAY faster than terminal velocity, and then begin to slow down the second they leave the gun. Rifling helps, but either way, a bullet that gets fired up in the air and then fell down and hit someone on the head would, more likely than not, just bounce off their skull, leaving a nasty bruise.

It is possible that if you fired a sniper rifle at a shallow enough arc, it could come down before it loses all its velocity, but then you have to consider the chances of that actually hitting anyone in a deadly place. And again, unless you're firing a sniper rifle specifically designed for distance, you still wouldn't have beat the longest kill record. And the kind of people who "fire their guns in the air" aren't generally wielding military snipers.