So, I got my hands on one of the new 6-cavity LEE C312-155-2R molds and set about modding it to plain base for PCing. Here's what I did:
I drilled out a 1" thick chunk of steel (happens to be 301 stainless but that's just I have a bunch) with a 5/16" drill bit (.310" with my caliper, very conveniently) This acts as a bit guide when reaming out the mold.
I chucked the bit backwards in my lathe and turned a shoulder into the butt end that matches the diameter of the gas check shank - .285"
I cut some crude reaming flutes into the shank near this new shoulder.
I added a depth stop collar to the bit to make sure I don't cut too deep.
That's it!
To ream the gas check shank out of the mold I do the following:
1) Remove the sprue plate and stop screws from the top of the mold
2) Carefully align the mold halves and clamp in my vise. I don't haul down on the jaws, just firm pressure. I don't want to smoosh the aluminum and ruin the mold.
3) Insert the drill bit shank backwards into the steel block
4) Align the bit shank into a cavity (the shoulder on the bit makes sure the bit is centered and the block as well)
5) Clamp the steel block onto the mold so it cannot shift. Now I know the hole in the steel block is centered on the cavity
6) Flip the bit around add some cutting fluid, and slowly and BY HAND spin the drill bit cutting into the cavity.
7) Clear chips with a Q-tip frequently.
6) As soon as I feel the bit catch a little, (meaning the gas check portion is almost gone) I remove the bit, clear chips and flip it around and ream the cavity out with the crude reamer. The depth stop ensures that I don't touch the lube groove.
7) When the bit spins freely, remove the steel block and using a razor blade, scrape the top of the mold flat to remove any burrs created by the cutting.
THAT'S IT! It looks pretty good. Once the bit is created, it takes a couple minutes per cavity to convert. The reamer function probably could be skipped with the cutting end of the bit used by itself and the stop collar adjusted for that side. Since I have the reamer, I use it, but it would probably work just great without.
Here's what it looks like halfway through. PB on the right, GC on the left.
Cheer's!