I've made many parts for musical instruments from G5 Titanium (Ti6Al4V). It's a light strong material, but perhaps equally important for airgun use, it's very easy to anodise. Once anodised it has a hard low-friction surface, the thickness of which can be accurately controlled by the voltage used, and can be added to at any time - it's doesn't have to be "baked in".

It's not as easy to machine as aluminium alloys, it requires sharp tooling and good control of tool temperature either through plentiful coolant or reduced speeds and feeds.

It is a poor conductor of heat.

If piston sizes are coming down to 21mm or so, maybe this is now an affordable option.