Sizing -- now that's a whole world unto itself when it comes to zinc castings. First off if I can get away with not sizing them that's what I do. If I must size them then it's a Lee type push through press mounted sizer in a strong press and it is best to heat the bullets to about 300 degrees first -- no joke, a Lee small size ladle pore pot turned way down works best for this. Zinc is very resistant to compression and not very malleable at room temperature with a very rigid crystal structure, heating them slightly up about the 300-F range makes them much more malleable -- that's a common characteristic with zinc alloys.
Lubing is always done separate from sizing if sizing is done at all. Zinc castings seem to have about the same amount of shrink factor as low tin lead alloys such as 30:1 with ''as cast'' diameters from the same mold being about the same with both metals.. I was intending to set the diameter of the mold to the diameter of the bore with the tolerances on the plus side with zinc, like jacketed bullets, you don't need to be a thou over bore size, you can be a thou under and they still shoot pretty straight. In addition zinc is slightly pickier than lead about fill out when it comes to narrow bands and lube grooves -- you'll notice how I'm keeping the bands and grooves nice and wide.
So anyway, at this point I'm basically considering working up into molds specifically designed for zinc casting. As far as the spruce plate holes; just like WW alloy zinc alloy really likes a big spruce plate hole. Think I'll be sticking with the larger WW spruce plates if they are harder to cut. First thing I had to do with my Saeco mold by the way - expand the spruce plate hole (used a counter sink cutter bit to get approx. the right angle) along with drilling out the gas check shank on the mold to make a normal base. Similar operation to what I'm considering except for expanding a hole with a drill press is pretty much a self centering operation. That thought does give me and idea though. I could use that same counter sink cutter with it's steep angle to center the drill press on the molds bore. First with the mold halves firmly clamped together and the mold sitting on the press table lower the counter sink cutter (motor off) down onto the mold bore hole, then clamp the mold in place, and finally raise the drill press head swap out the counter sink cutter for the ball end mill cutter. Theoretically, that would make sure I was centered dead on and not just be trying to eye-ball it. Got to order a batch of cheapo Lee molds and the ball end cutter and start doing some testing . . . .
It might be some time before I actually put in the order for the first mold specifically designed for my zinc casting. Need to work all the bugs out of this round nose idea first. I think you see where I'm going though. Nose profile built for high speed full power load velocity, mold that drops bullets at bore size (in this case both my 303-brit and 7.62x54R are same so two guns with one mold) or just slightly under or over by a thous so no sizing, large size holes in the spruce plate, and nice wide body bands and lube grooves for proper fill out especially an extra thick base band with no gas check shank.