I am loading a Uberti 1858 Remington revolver and am using a .45 conversion cylinder and am using a Lee 255 gr mold, when I cast using 1:20 or even 1:16 my bullets are in the 269 gr range using the 1:20 and 263 gr range using 1:16, I want softer bullets that weigh in around 255 gr and pure lead is to heavy for the molds I am using but if I can cast bullets using the 1:10 for a hardness of 11-12
and they will soften up to about a 8 bhn in a month it should work out great at my 750 fps velocity using 30 gr of KIK black powder.
THe thing to remember when using almost any commercial made mold is that they are cut for a specific alloy,(ie. Lyman 2 or similar). When you pour using straight WW alloy or a variation of it, your bullets WILL drop heavier than what the mold usually say's they will. Most of my 255's from the Lee mold drop at 260 to 270 depending on the alloy. I simply drop the powder a touch to cover this and all is well.
You would have to add in some antimony to get your weights closer or you might look at a custom mold where you give them your preferred alloy and they will cut the mold to drop the proper weight bullet. Not the cheapest answer to your question, but if you want a 255gr bullet using your alloy about the only way to get there without adding in more antimony for a harder bullet. The closer to pure you go the heavier it is going to drop.