I read several years ago on castbullets.com about enlarging
The cavity by lapping the cavity of a mould with lapping 
Compound and a casting from said mould to inlarge it. I have a 2 
Cavity steel Lyman mould I would like to inlarge 1 of the cavities from .321 to .324 to see if it will work better for a 32-40 I have.
Has anyone on here ever tried it? And what might drawbacks be?
Any feedback would be appreciated .

Thanks doug

Certainly possible.  Has been done many, many times, including twice by me.

Assume when you say "steel" mould, you actually mean cast iron.  Cast iron laps easily, steel does not.  The difference is that cast iron is a friable material, from which tiny fragments can be removed with low energy of dissociation.  

Only "tips" I have to offer are 1.) Drill a small hole in your lap, (on center of course), and press in a Torx bit to use as the driver.  The old recommendation of using a screw doesn't really work all that well, since you should be reversing the direction of rotation frequently.   And 2.) Don't use a lot of lapping compound.  Spread a little bit on a flat surface, roll the lap in it using another flat object, using some pressure, so as to impregnate the lap with grit. Gently rinse the lap in solvent to remove excess before going to work. 

Oh, and don't turn the lap fast.  A power screwdriver is OK, your electric drill is not.  Use a very long extension to avoid putting side thrust on the lap and thereby making your cavity oval.

Doug, the boring in a Metal Lathe is the way to go. I am sure if you poke around the folks you know you will find a Machinist guy who has a 9" South Bend with a four Jaw chuck. The only Boring you should have him attempt
is the Base Band. I have some number of "JUNK" molds that represent my efforts to go into the Mold and make changes in either the Nose Shape or inner diameter. Finally figured that was not going to work in a Manual lathe.
What you should do is cast a small quantity with your Alloy and measure them
up and Pin Gage the Base band diameter in the Mold. Knowing what they Measure as cast and knowing what the Base band measures gives you a close figure for shrinkage. Now you can decide what you think the Base band diameter should be and bore accordingly. This process has worked very well for me for
30 Years or more and I have been able to hand back bullet Molds that then cast very good bullets that shoot well. HTH FITZ. OLD TUCK

I have to agree with Doug (OP).  Everyone on this site doesn't have a machine shop at his disposal for this sort of thing.  I for one certainly don't.   If I wanted to open up a mould a few thousands I would cast a bullet of wheelweights or #2 alloy, drill a holl in a slug, put a small torex screw into it and give it a try.  If it works, great.  If it doesn't you still have the other cavity to work with.   I once lapped a 58 muzzle loader with a hard cast minie attached to a ramrod with a screw and using valve grinding compound.  Worked great.  Later when I traded that musket off the fellow who got it was winning all sorts of matches with it.  Having a complete machine shop is great, but the reality is that most everyday shooters don't have one.

Keep your powder dry!
George

Enlarging bullet mould cavities can be done by lapping with succeedingly larger castings, but in my experience, there is a law of diminishing returns that says +0.001” is easy; +0.002” is doable, but laborious; +0.003” is at or beyond the outer limit of possibility.  Trying for that much or more may eventually get you your desired dimension, but the bullet cavity will become noticeably oval across the casting line.

A regular production mould is likely somewhat oval already, so this condition may or may not be bothersome to the end user, who sizes the bullets in a lubrisizer.  Those who insist on zero runout may get more upset if their expensive custom mould is a half-thousandths fatter across the parting line at the end of a lapping session.

Lapping ovalizes bullet cavities faster per thousandth diameter increase if the mould is held tightly in handles, but some runout will occur even if the blocks are fixtured in a four-jaw lathe chuck, and the excess lapping compound and swarf cleaned off the faces frequently.

I usually put the bullet in a collet, drill and tap it with some appropriate-sized tap, and leave the tap in the lead as the shank.  If I use an electric drill, it is a variable speed one, at very low rpm.  Thirty seconds or a minute slow turning, open blocks and clean faces, the same time more in reverse, clean mould and cast a few to check diameter, and, if necessary, supply the next size laps.  It is, of course, more involved with the four-jaw chuck, but the buildup must not be allowed.

Update;
Gave it a try the 2 cavity mould was already 4 thousands out of round , lapped it , I did get it to .324 one way but now it is
6 thousands out of round. Oh well live and learn. I still have the other cavity that is only used for a lever gun I do not shoot much.
Now I need to do a chamber cast to see how large of a bullet I can fit in this chamber. Because when I slug the bore I come up with
.326. standards say I should shoot .327 , don't know if that will chamber . Another thing is to try 30-1 instead of 20-1 for more bump up. It's an all original 441/2 Stevens in near perfect condition. I would just like it be able to shoot  1 to1 1/2" moa. It will not be used for any competition.
Well on to the next step.
Thanks for all the feed back

Doug

Silver, how are you measuring roundness.?  Those numbers sound absurdly bad to me.  

And .326 sounds awfully big for a .32-40 barrel, even for Stevens.  Are you using a micrometer or a caliper?  Is it properly calibrated?

n.b. I lapped an aluminum Lee Bator mould from .225 to .228+ to get bullets for a .22-20 project.  I just took an as-cast bullet from storage; it measures less than .001" out of round, using my best 1" micrometer.   What did I do wrong?

I’ve never lapped an aluminum mould, just the iron alloys.  Aluminum may be more forgiving, since it would obviously cut easier and faster.  It would be interesting to know how big an aluminum mould cavity could be lapped to before it started going oval.  I would think it would eventually.

If you have a good sizing die that swages the bullet shank round, there may not be any indication of a problem in the actual shooting results.  If castings from a custom mould are pan-lubed, the bullet will remain oval, but of course will round itself quickly enough when it goes through the leade and barrel.

There was a fanciful discussion in one of the old American Rifleman magazines about how sizing and shooting couldn’t help out-of-round bullets that much because the noses and the bottoms of the grooves would still be oval.

I haven’t noticed any effect from out-of-roundness in my shooting of all kinds of bullets from all kinds of used and abused (and reconditioned) moulds purchased at gun shows, but I’m not all that great of a shot.  The smallest five-shot group, with whatever components, is The Load, as far as I’m concerned.  With me as the big variable, I at least have the luxury of not having to flip out over such tiny nuances.