It still won't be 100% soft like you want. But you can dilute it with soft lead, that seems to work for me just fine. I found that the salvage yard a few towns over sells soft lead for $1 a pound, but it's good because unless I make shot from it I can re-use the rounds I find in the berm. Airgun lead is pretty soft, almost pure lead, I just bought an airgun for cheap backyard plinkin' and for killin' pests. I also have an airgun pellet trap and a berm I set up that should return a fair amount of lead to me. That's good because I shoot and then get free ammo for the rest of my guns (sept for the pellet gun and rimfires.) Another good way to get soft lead is from jacketed bullets, the copper jackets have soft lead inside, you need to break open the FMJ ammo with an axe, the cast bullet are generally hard, rimfire is medium hard but will work for anything but Minnie balls, those should be dead soft. For buckshot it doesn't matter much, patched or saboted projectiles could be harder if you need to get rid of the hard stuff. That's what I do, it works just fine. Anyway, that's all I have to say about that.
~Levi