If a Guncutter uses Pilot [spaceship], why shouldn't Mastery of Space count towards re-rolling for that?

Because a guncutter isn't 3km long?

Because gun-cutters are vehicles far to small and maneuverable for Pilot (Space Craft) to be relevant, and Pilot (Flyers) should be used instead?

Darth Fanboy said:

I'd like to see you pilot a jet fighter in space.

I'm not proposing that Mastery of Space extend to atmospheric actions, but if you're piloting a guncutter or similar smaller space vessel IN SPACE then why not?

Also: Into the Storm classifies guncutters, shuttles, etc. as Spacecraft, not Flyers. Basically anything that is SSTO. This means you use a Pilot (Spacecraft) skill for it.

Errant said:

This is basically the answer. Mastery of Space is not a mastery of vehicles that happen to move in space, it is a mastery of the massive warp capable vessels that are used by Rouge Traders and the Imperial Navy and such. Mastery of Small Craft involves piloting... well, smaller craft. It's kind of like being good at driving the NASA Crawler doesn't mean you're good at driving a tank just because they're both tracked vehicles. One is hugely crazy massive and requires an entirely different skillset to work with than the other.

Presumably, driving a tank isn't the same as flying an aircraft, but Mastery of Small Vehicles gives you a reroll on both. AND Guncutters. Essentially anything that isn't a capital ship!

True, but tanks and speeders are much more similar than cruisers and guncutters.

Mastery of Space applies to Maneouver Actions aboard a starship (or space ship). It does not apply to any given roll of Piloting (Spacecraft). In starship combat these might as well be the same thing, but at other times a simple test of Piloting (Spacecraft) might be required (see Space Travel Outside Combat, pages 226-227 of the RT Core) and Mastery of Space does not apply.

Mastery of Small Craft applies to any given roll of Piloting (Spacecraft) so long as the vehicle is a 'small craft' (vague definition most certainly excluding starships). This includes both in combat and outside of it, but note that small craft don't make Maneouver Actions per se.