You are absolutely correct. Lead will help your barrel last, and the milder powder charges used when shooting lead through modern rifles will also extend barrel life (the more powder you use, the faster it erodes the lands). The one exception is salvaged range lead or wheel weights that are not properly fluxed and skimmed. They can be so full of grit that they will lap your barrel into a nearly smooth bore in short order. It's easy to avoid this, just flux, mix, and skim.