Made up a set of right angle "Z" shaped shims for working on molds. One lip hooks the base of the mold. The other presses against the face of the chuck jaws. Holds the mold securely without excessive pressure. Shim machined as one piece, then cut into four pieces.
Chuck

Chuckster wrote on Feb 23^rd, 2014 at 9:31am:


Makes sense.  Facing the front of the jaws first (to accurately locate on them) is likely a good idea too.

What material, about how thick?

Chucking a lead tin alloy bullet in a lathe chuck without changing its true balanced form is in my opinion virtually impossible and then to use it as a lap spun by a hand drill with the intent of ending up with a larger yet concentric and balanced bullet is asking a bit much. There is no doubt that one can lap out a mould to enlarge it, it is indeed a very simple process but I have yet to see one end up being round let alone being balanced.

JLouis

This all may be true but, and it's a big but, I have lapped out a mold with the result that the bullet shot a lot better than it did before I lapped it.  If you start with a mold that's not much good to begin with, what do you have to lose?  If I want a really good bullet I order a custom made mold.  How many production molds have you seen that cast out of round bullets to begin with?  I have a few and in addition to being out of round they are also often undersized, at least with wheelweight alloy.  I have even sent one back, to one of the oldest makers of molds, only to be told that the mold was "within their specifications."  It was "only" 2 mils out of round as well as being undersized. It all depends on the use of the rifle and load and how accurate it needs to be.  The many rabbits and grouse I have shot with my 30-06 didn't seem to notice that they'd been shot with a bullet from a lapped out mold.  What's a poor boy to do but make the best with what he has?  By the way, I printed out a copy of that 200 yard group you posted and hung it on my shop wall as a goal to achieve.  And no, I don't expect to beat it with a lapped out mold!

John Shaw 

To try to lap out a mold 4 or .005 would be a lot of work. I'm a lazy machinist and would probably sell it and buy another but, for the ambitious, it can be done.

You would not put the cast bullet in a lathe to drill it, so you can put a shank in it. You would insert the bullet into the mold and use a filler such as rosin or even some kind of powder to take up the clearence in the cavity. It would also help to cast it of Linotype so, it will cast larger. You would then drill the bullet in the mold, held in a precision machine vise, in a milling machine (safest way). 

File a length wise groove in the bullet to hold the compound and proceed as usual,with lapping compound and like one poster said, make sure you keep the block half's clean of the compound.

Lapping WILL make a mold round, as the highest points in the mold will get the most or all of the pressure, depending on how out of round it is and as it gets round the pressure will equalize.

Frank

Have used various grits of lapping compounds rolled into shaped leaded screw stock (similiar to how some prepare bullets to be used for fire-lapping) to bring  some special non-standard cutting bits to final size and polish.
As Frank says, it is slow, but slow can be a very good friend at times.
Might be able to make a bullet nose shaped screw-stock lap (no soft lead/tin alloy to be distorted) with a slightly shorter and tighter nose radius shape (with the compound carrying slot), to lap/polish the rear section of the undersized bore-riding area of the mould.