Fella what wants to cast pure lead ought to jack up the temp to 800*. It'll fill out the mould. Been doing that for a fair while, no problems.
As suggested previously, bullet fit is more significant than alloy, to a point. The type of powder is relevant if one is trying to bump up the velocity as is lube, be it powder coat or whatever. I'm not a PC kinda guy, my pure PB bullets seldom travel more than 1100 fps, paper patched or lubed.
I do run 30:1 alloy upwards of 1800 fps with a gas check and 1400 fps or so without, BP and smokeless both. .25-.50 caliber. The .38 and smaller bullets used for smokeless get my own brand of dry wax lube which is dilute Alox and a dusting of mica/graphite before fully dry. The RCBS .38-150 SWC dribbles out of my 2" Smith around 900 fps with a bit more than average charge, this with the same alloy. Bullseye does the trick.
I don't do HP bullets anymore, but if so disposed that facet of design would have little influence on my chosen path. Got over that when I began choosing placement and penetration over hole size. Soft slow bullets don't hardly slow down for anything.
There was a time however when I was inclined to load the Hornady 148 gr HBWC backwards and forwards. Backwards will decapitate a 4' barracuda if a fella wants to retrieve hooks'n lures expeditiously. No leading from that soft alloy up to about 800-850 fps in the Smith.
