Took this off of the muzzle loader site I go to after posting about Sumners rifle problem.
Hard lead will lead with low pressure loads, such as black powder loads are, where pure lead will not. It is indeed because the hard lead does not "set back" and seal the bore, and the gas goes past the projectile, melting the lead as it does.
No doubt those REAL's you had were cast from harder lead, someone probably "meant well" thinking they would work better for heavier loads....NOT!
I cast my own REALs out of PURE lead and they work very well, ZERO leading with heavy loads. REALs are good bullets, but must be cast of very soft lead.
The harder the lead, the more pressure it needs to set it up a bit. But even the most heavy loads (within reason) in a muzzle loader don't approach the pressures needed to make the very hard lead alloys work. Nor do they produce the velocity for soft lead to lead a barrel, from velocity in it's self.
Also, in a modern gun/breech loader the bullet is usually larger than the bore, which makes the problem less severe.
No leading will hurt a bore if it's cleaned out properly.