[oil pan 4]

You end up with low sectional density and high ballistic coefficient bullets. They will be more difficult to stabilize.
There are more and more solid copper options and solid copper is usually a drop in replacement for copper and lead bullets.
I have found the solid copper bullets shoot great, only disadvantage is they cost at least twice as much as lead. That's just the bullets, everything else costs the same.
I use solid copper as my brush bullets as I might blast a mule deer from a few meters away in stand of brush and trees. A solid copper bullet won't fill the deer with pulverized lead particles. I actually was going to switch over to solid copper but I can't reliably find those copper bullets.
For longer range like 200 meters and up I use lead bullets. After a brief flight and a little aerodynamic heating the bullets slow down enough to not uncontrollably fragment.