Your plumbing lead is basically pure lead, and will work for lighter loads, what most consider target loads, under 800fps, but is not ideal. For best results you would want to add some antimony and tin to what you have.
It doesn't have to be too hard...you can mix it 50/50 with wheel weights and be in the ballpark, or better 2 parts wheelweights to 1 part lead.
I powder coat, but that alloy won't lead if conventionally lubed as long as they are sized correctly for your firearm.
I keep some pure lead around for black powder, and various lead alloys in ingots and blend in the pot.
Having some pure lead around is handy for black powder or to alloy with lead that is too hard.
