For the 20 or so years I have been casting, I've either used wheel weights or Lyman #2. Both are hard enough for pistol loads, and even many rifle loads, especially with a gas check. But lately I've been casting big boolits for black powder cartridges, so I decided to grab some 20:1 so it would be softer.
Normally, I set the mould on top of the warmer while the lead is getting up to temperature. Even so, the first few boolits I drop out go right back into the pot. The mould temperature simply isn't hot enough to cast without some wrinkles. This even works with my 500 grain .45-70 boolit.
However, with my .50-70, the mould simply wouldn't heat up enough. I kept getting wrinkled boolit after wrinkled boolit, no matter my cadence, etc. Wrinkled boolits generally indicate it is too cool, so after I couldn't get the mould warmed up enough, I turned up the dial on my pot to 800. I don't have a lead thermometer; I simply make adjustments to the pot based on what is dropping out of my mould.
This seemed to do the trick. I got a nice batch of boolits for my antique .50-70.
Has anyone else noticed 20:1 lead has to be cast at a higher temperature than harder alloys? This was also the first time I had used that mould, so perhaps it is something unique to moulds that large?