..............DanishM1Garand, I did what you were suggesting for one of my customers several years ago:
By the way, you do NOT have to buy a boring bar for this job. If you do much of this type thing you'll find that fairly inexpensive drill blanks will become your friend, as below:
You can grind your own boring bars/form tools/spoons, or whatever you want to call them. I do not have any fancy grinding equipment. I have your standard bench grinder, a Balbor type grinder with a diamond wheel, and a set of diamond files, and an assortment of stones. All the above were made from drill blanks which are simply HSS sticks, ie; A drill bit with no flutes. They were also made before I had the Baldor. Just takes a bit of careful and thoughtful work.
More importantly, doing work like inside those mould blocks means you cannot see what you're doing. You'd mentioned having access to a lathe. Since I have no idea what it is, or how it's equipped I'll simply relate how I did it on my 11" Logan. After taking measurements of the mould cavity, I had to produce the tool to cut the drive band widths to what I wanted. Measuring to the depth required from the base of the blocks, measuring the "DEPTH" from the ID of the core pin bore to the ID of the slug + adding in the depth of the cut required to produce the drive band + a bit of clearance through the core pin bore.
This was all measured and then added to a drawing of the required tool. After it was committed to paper it was then double checked to make sure I hadn't forgotten anything. Since my lathe doesn't have a DRO, long before I'd fabbed up fixtures to clamp to the bed and the headstock, by which I could attach a 1" or 2" DI which would bear on the carriage for X axis feed. The compound was going to be used to feed the tool on the Y axis (depth of cut) so a another DI was attached to the cross slide to read on the compound.
BTW, your tool MUST be SHARP, as it is of rather small cross section, it's 'Stuck out', and you're cutting on a comparatively broad face. You take small incremental feeds on the tool when cutting. Also when setting up, the tool MUST be centered on the work. Too low and the tool may want to grab, too high and it won't want to cut. Normally cast iron is cut dry but I used WD-40 and it worked just fine.
You won't really be able to hear the tool cutting, but if you just 'Lay' (barely touch) a fingertip on the tool shank when you crank the compound out, you'll feel it begin cutting (faint high freq vibration) , and you'll feel it stop cutting. I don't recall now how much I fed to cut, but it wasn't much. Maybe .003 - .005", don't remember. Just be gentle to start and remember you're NOT in a hurry. Since you're hand grinding the tools, if you bust it your chances of creating an exact duplicate will definitely be problematic.
The large photo of that Medieval looking form tool was used to make a swage die for these slugs for paper patching:
The right photo getting ready to make another pass into a swage die body. Even though that tool is a monster compared to what you'd used for that slug mould, it has a huge cutting area as it cuts from the tip to the heel, so small cuts and slow feeds are required to prevent it flexing/vibrating. As you can see from the finish on the swaged slugs, if sharp they'll produce a very fine finish.
When I got a rifled bbl for my Mossberg 500, I wanted a full sized slug (above) so I made a mould to cast the above slug. It weighs 525 grs. I filled the base of one with hot glue just for fun and IIRC it weighed 532 grs.
After making the blocks, they were chucked up in the 4 jaw and the cavity was cut using the 'Ho-Made' form tools. On the right is the blocks assembled complete with a captured core pin.
A set of Quick 'n' Dirty blocks and a cavity bored for a 20 ga slug, again using simple ho-made form tools.
..............Buckshot