I kept snapping the ends of the reamers finished up with loads of them , collets helped a lot to keep everything lined up .
I broke one D bit, at my first attempt I was gripping it in the drill chuck, so a swapped to using them floating I had no more trouble after that.
In the tool room where I served my apprenticeship, we use to make 400 plus bullet dies a week (that's bullets as in bang) the dies were tools steel and they were all cut using bullet shaped D bits and they were always used with a floating holder, and we had a oil stone to keep them sharp, you could make a batch of 50 with just one D bit, these were ground up from tool steel, we had our own hardening shop so the dies and D bits were hardened and tempered in house. Now the bullet dies are from tungsten with the form spark eroded in.
Stew