In a properly made piston, there is no contact between the cylinder and the piston itself. Just the sealing rings and the bearing surfaces touch and those can be made of good plastic or metal (look for "buttons" in all airgun fora. Neither the sustained temperatures nor the pressures are such that tool grade metal or other sophisticated material needs to be used.
Good quality cold-rolled steel will do if you want to use steel, but in a 45, I think it would be very interesting to make up an aluminum piston and make buttons and a ring to seal the whole thing.
This would yield a very light assembly, ideal for light pellets. If the piston is light, then you can add weight into the piston in several ways to "tune" the weight of the piston to the weight of the pellet you want to use.
If you want to use heavier pellets outright, then you can make a steel one.
Stainless could also be an interesting possibility, 316 could provide a very fine finish, while still being reasonably machineable.
I know I am not providing hard solutions or suggestions, but in reality you have an awfully big sand box to play in, so, just try several things and see where you want to go.
Just keep us posted!
Un Abrazo!
Héctor