yep back in WW2 the cap badge for the FS hat and coloured FS hat and the ridiculous GS hat was the Bomb in either Brass or green or Black Bakelite. And OR's also wore the coloured field service hat. On the service dress hat they wore the Gun badge.
But there are many pictures of gunners with Gun badge on FS hat. But then with all the converted Yeomanry and Infantry units in the RA there was alot of tribal headdress.