You are only concerned with pure lead when loading, especially if you cast your own round ball from a mold, because lead alloy as you are harvesting from the range scrap, does not contract as much when it cools after being cast. The round ball molds are designed to take this cooling and shrinkage into effect, so that your .530 mold will give a ball, when cooled which is .530 [give or take one or two thousandths] in diameter. THIS is then coupled with a patch for a rifle, and this combination is
tight. Now if you then switch to the alloyed lead, you might find as others have found that the alloy ball and your standard patch are now too big to load into your rifle.
The shooter then must find a thinner patching material, OR a mold that casts a slightly smaller ball, when using the alloy..., that's all.
It's usually not as crucial with a smooth bore if one is patching the ball, and if not patching, and the ball fits down the bore, it's no big deal. As for performance after impact..., yes soft lead will deform more, but considering that you are probably when shooting a smooth bore, using a 20 gauge, that's a .600 ball or larger in many cases....that's a pretty big hole even if it doesn't mushroom or deform. HECK a .530 ball launched from a smooth bore 28 gauge is a pretty big hole. The deer will probably be just as dead.
Leading.???
Seriously folks those of us who shoot shot in our barrels without a plastic shot cup, or any shot cup, find no leading in our barrels. I think the surface area of the shot column bearing on the interior of the barrel is a lot more than the surface area of a single sphere bearing on the interior of the same barrel. Not to mention, the shot loads are normally in excess of the volume and mass of the lone round ball in the same gun. Jamming conical projectile onto the lands of the rifling when firing a that projectile from fixed ammunition in a breech loader, even when using BP, has a bit more going on with the projectile and barrel interior than we do with our traditional guns and rifles.
LD