.44-77 PP bullets.

[Distant Thunder]

Sam,

I learned tool & die using O1, A2 & D2 long before the powdered metals, CPM 3V, 10V, PM4 and others, were being used. I never liked O1, so I have used more A2 & D2. It's just really hard to beat the air hardened tool steels for ease of working with and dimensional stability through heat treating.
For a sizing bushing I would probably use D2, I think A2 would work fine too. I use a lot of A2 because it is so easy to work with and heat treats so easily and I have a lot of little pieces of scrap left over from my working days. D2 would be a bit better choice because of it's higher chromium content would polish very smooth.
Basically I use whatever I have laying around that is closest to the size I need and will do the job.