For muzzle loaders, the recommendation is always to stick with pure lead.
For breech loaders, some alloy of some sort is the recommendation.
For low-velocity loads, why not use pure lead in a breech loader? Some of the muzzle loader conicals approach 1500 fps and no-one gripes about leading. Pure lead flattens better, giving more energy deposit thus making milk-jug targets more exiting :-) .
Maybe the pure lead just doesn't work with smokeless?
Obviously this has been tried before otherwise we wouldn't know about the leading problem. But maybe the leading only becomes a problem at certain velocities or with certain powders.