Hi there can you use 308 brass to make a 338 Federal round? And if so would fire forming them be the way to go ?TIA Nim

Just run them thru a 338 Federal sizing die and make sure they don't need trimmed.

Would replacing the expander ball with an 8mm unit make things a little easier for a first step? I know you can do this with RCBS dies, not sure with other brands.

I've upsized in one step from 308 to 358 without any problems, except for some real old brass with brittle necks.

Lube it good and go slow. An intermediate step might give longer neck life...dunno.