I just discovered that www.buckshotmold.com/
has a variety of buckshot molds available for casters. They have a 0000 Buck mold that casts 10 (TEN!!) .380 diameter buckshot per casting (and 20 shot per cast in smaller sizes). Now, this isn't a precision mold like you would get from Lyman, RCBS, Shiloh, LEE, BigLube, etc. but the nominal diameter is .380 inch (I'm guessing + or - .003" variation). The casting cavities are machined not molded into the aluminum molds and wooden handles are integral. It is not the cheapo sinker mold you see at the sporting goods store. The mold costs $34.95+$5 shipping. They even have combination sizes like my mold that casts #4 and 00 buck. My thought is the 0000 Buck mold should cast round balls that ought to work just fine in the various 36 cal Colt & Remington replicas at the short ranges we use in CAS. For precision shooting there may be too much variation in ball size for real good accuracy in target work. For most CAS shoots where the targets are 7-10 yards or for plinking they should perform well enough. If I were ever to go to EOT or Winter Range I'd use factory swaged balls (or sorted & graded hand cast from a LEE or Lyman quality mold) but for monthlies these oughta be just fine. I do not work for Sharpshooter USA that makes the mold. I am not getting compensated for this "plug".
I just bought their combo buckshot mold and it works beautifully once it is heated up. I leave a full length sprue attached before nipping off the shot. That warms the mold faster and leaves bigger chunks of lead that are easier to put back into the melt while casting. You'll need a flush cutting nipper to remove the sprues (@Harbor Freight for about $2.50 or $5 shipped with the mold). I just wanted to let the pards I shoot with know that there is an economic and suitable mold out there for volume casting to help save a few bucks on ammo for those 36s. Their #0 buck mold (.320" dia) might even work for the 31 cals. If you have already tried this mold and it didn't work for revolver shooting, please let the me/us know. All I can say is their buckshot in shotshells patterns just as good as factory loads but are a whole lot cheaper to make yourself.