I believe I know why AndrewB said what he did, but I will have to look it up to be sure.
A bullet is a projectile that is fired from a “rifled” barrel. Since the projectile must be malleable it precludes the use of iron.
Now I’m off to see if I got this one correct.
Ok, from reading a couple online definitions of “bullet” it seems that it is possible to shoot one from a non-rifled barrel as long as it is round.
So I guess it is more likely that steel bullets were never used what with the availability of lead and all.