Not to mention that a 50 gr. bullet weighs 50 grains weather it is 100% lead or 100% copper. The shape of that 50 gr. bullet needs to be concidered more so that what it's made of!
If you had 100 pounds of feathers compacted into a small ball like shape and 100 pounds of lead and drop them from the same height at the same time, they both should hit the ground at the same time, as long as the feathers don't have more wind resistance than the lead does.