Prop Shooting Gun Auto Cleanup?

iam working on a Gun"Pair of hands that trows a rock" i can get it to throw the rock and i can remove it with the undo key but is there a way to have it undo the rock by its self?

timer.Create(rock:EntIndex()…"_remove",3,1,function(ent) ent:Remove() end,rock)

its giving me attempt to call method ‘EntIndex’ (a nil value)

ignore this guy he doesnt know anything