Thanks for that longbow. Much appreciated! You have given me an idea. Mmmmm.....!
In the meanwhile, I have 'unstuck' my trial mold and made a casting which I am about to patch up and see just how it will fit my rifle's throat. My next step is going to be to machine dummy 'boolits' with a long shank that I can trial by holding the shank and inserting into the chamber. That's just to determine what would seem to be the ideal prime casting shape to achieve best fit after patching. Then I shall poceed with the idea you have just given me (that being how to make 'D' reamers quickly and easily).