IMO, it will never return with the same velocity as when it left the muzzle if fired in ‘overhead’ fashion. It depends on the angle, which is what I’m taking from your explanation.
Yea, it’ll lose velocity. All opjects falling have a terminal velocity where the effect of gravity is equalled by drag against its fall. For a bullet shot straight up it’ll hit an apex, stop, then fall back in tumble never picking up enough speed to easily kill.
When you fire it in an arc, it maintains horizontal velocity and it never comes to a full stop in flight. Instead it maintains a fairly stable trajectory and will hit much harder and can possibly kill.