1st 6ARC loading.

[grayfox]

As a rough estimate, you can use hornady's data for their 75 gr Vmax, the switchback is 22" bolt action and the bolt action data is 24", plus an 80 gr bullet should be about 50 fps slower than 75 gr data, grain for grain of powder. The Vmax has benchmark listed. So I would say maybe 100 fps slower than 24", 75-gr bullet data. give or take.
Let us know how it turns out!