I'm making my own bullet mold using 7xxx aluminium (for the ease of machining mostly - also I like aluminium molds). This mold is a replica of a 1851 Swedish Kammerlader for Navy bullet.
It is a black powder breech loading rifle. The bullet will be cast from pure lead. The target diameter is 0.610 in (15.5mm).
Can someone who made their own molds advise me on sizing the cavity, please?
Should I simply calculate it based on thermal expansion of aluminium at mold casting temp, both expansion of the mold and contraction of lead or is there some rule of thumb for sizing cavities?
Currently I'm halfway done with the block. I have the alignment pins set, the inside mating surfaces ground and the air escape channels cut. I'll be lathe boring the cavity by holding the block in a 4 jaw chuck.