As rig no's change every week it seems, would you be referring to shirt and tie / tropical shirt? If so then I don't see why not ask a FOSTie as they have been doing as long as I can remember.. I've never been picked up for it but some old timer SR's might see their arrse
WOs SRs and Officers (staff and students) at Sultan were not allowed to wear berets other than for range stuff last time I was there. Huffy dits in Daily Words and bollockings a go go from the Commander, Jimmy, WO Joss and WO (M).
Berets are 'kin awful unless you are doing range/infantry stuff. Baseball caps are worse.
Its all in the shrinking to fit (not pleasant wearing a wet beret for hours until it dries).
As a matter of interest:
When I shot in the Inter-Command Rifle Meetings in the late 60's - early 70's, we wore full No2's, Gaiters and Cap! But we were allowed to turn the cap around so the peak was at the back whilst actually firing!