Agreed, wheel weights, linotype, monotype and the like all have both tin and antimony in them that will water harden somewhat. Nearly pure lead though, will exhibit very little change...at least in my experience.
I should add that I made all of my PCP airguns myself using TJ and LW barrels. I cut my own chambers and throats in my airguns. Big bores (.308, .357 and .458), are always enlarged (lengthened) over what I might use for a pellet chamber, otherwise, yup, you'll never get a slug in there, hard or soft.
I do not own an Airforce or FX or...
I do have a Marauder but have not re-barrelled it...yet. And I only shoot pellets from it. I tried .25 slugs in it and did not like the results. Same in my self built .25, pellets just seem to work better in those, so I stick with them.
Chris