new caster

44 joe, generally speaking, most molds are cut .002-.003" oversize. then they are sized down.
some molds still need to be opened up more than that. it depends on the bbl quality and slugged size.
most people use wheel weights, some use 20 lbs of wheel weight with about 1/4 -1/2 cup of chilled magnum shot to help with the mold fill out and hardening properties of the alloy. then water quench for a really hard bullet.
zinc has proven to be a real pain in the tookus for casting bullets. the flat, nearly silver plated looking weights. they are very stiff and hard. generally if they flatten fairly easy, they will make good bullets.
hope this has helped, if you are still interested, order Marshalls book on casting etc. it is a real good start.
hope this has helped, halfbreed

44joe
Bullet molds almost always cast a few thousanth's over nominal diameter and then you size them to your requirements. Yes custom mold makers will cut you a mold to cast a specific diameter but only from a specific alloy. I use alot of wheelweight's and sometimes add pure tin or lead to get the alloy I think I need. I get mine from tire stores for any where from free to .15 per LB. After you melt them down and flux real good they are ideal bullet alloy for many purposes as is.
Hope This is some help to you.

Thanks for the advice. Marshall's book is on the next list as is Veral Smith's. I will be visiting the local scap yard looking for the dull soft wheel weights. I've seen ads for lead and alloys in handloading magazines, but I don't like to pay full price for something I can get for less.

44 joe, also take in consideration the shipping charges.
and nearly any tire store will give you all their used wheel weights. halfbreed

check out the local printing shops to some still have linotype laying around that theyll give away or sell cheaply.