"pure lead" probably isn't but wouldn't make any difference re bullet casting. "Pure's" gonna require a bit of alchemy.
Grab a copy (any # really) of Lyman's cast bullet handbook for all the ins & outs re alloying for bullets. More info really than most of us care to know ...
In my experience, .30 cal require that I do add the tin for castablility OR, jack the temp up higher .... same temp, same alloy as for .38/.357 & I just don't get the mold fill-out.
Straight wheel weights have worked very well for .308, 30-06 & 9mm/.38 for me with the above caveat re tin/higher temp for the .30s
Although the cats can kill just as well as a jacketed bullet (ask the buffalos), they're not in the same league as far as speed goes.
However, they can (it'll probably take more load development & work) be as accurate - at least in the 100 yd range.
If you try the bullet hardening by dropping them into water - PLEASE - be very careful with water around your liquid lead. Not a very good mix & water hitting that lead will instantly turn your water to steam which may blow hot lead a ways. Once usually does it.