[SR4] Build/Repair for weapons

Hi!

How do build/repair-skills work for weapons in SR4?
In the conversion-guide I can find stuff for vehicular-skills but what
is with weapons?
I have a player who wants to play an expert in explosives and weapons.
For using explosives and building his own bombs and grenades I guess
he needs "Demolitions".
What does he use for repairing guns (all types incl. gunnery)?

Niels