Weight of the bullet has nothing to do with it. Physics states that distance traveled X equals initial velocity * time + 1/2* acceleration * time^2

X=Vo*t+1/2*a*t^2

Because both bullets start at the same height (and assuming they fall the same distance) They would hit the ground at the same time due to the fact both would accelerate at 9.8m/s^2 (remember that the fired bullet has no y-axis velocity at the start only x-axis). To be very technical about it due to the curvature of the earth the bullet fored from the gun should actually hit later because a bullet shot at a tanget path from the earth would end up slightly farther away from earth than at start.