155 ELD-M
155 Scenar
155 Sierra TMK
As fast as you can go, hopefully north of 2900 fps
If that's not doable out of a 20, try the 168 ELD-M or 168 TMK as fast as you can go.
I have my doubts about being able to push anything heavier than that fast enough to make use of their higher BCs, but I could be wrong.
To be completely honest I'm leaving the 175 class behind for the lesser drop of 155s going really fast out of a 24" tube.