my brother in law wants a Wilson combat in 9mm. it's not on California's roster.
Single Shot Exemption is probably the answer - see the long thread at Calguns
As you seem to already understand, the issue is the Roster.
A handgun not on the Roster may not be transferred into or in California unless
* the buyer is exempt from the Roster (principally LEO)
* the transaction is exempt - intrafamilial (child/parent/grandparent - brothers and cousins and whatnot are not included) or Private Party Transfer (2 parties with CA ID meet at a CA FFL)
* the gun is exempt from the Roster (one example, Single Shot Exemption)