In order to Single Shot Exempt a handgun that is not on our handgun roster, the handgun needs to be converted to a single shot pistol.
These features are to include, a barrel over 6 inches and a magazine block, essentially turning it into a single shot pistol. (Single shot pistols are exempt from the CA roster). A magazine block is usually just a block of wood.
The hardest part for most people who want a more rare pistol, is finding an extended barrel. A couple shops in CA are thankfully doing the P-09 now. I pick mine up on 2\8. More common are the glock gen4's, XDM and XDS, some HKs and certain 1911s.
There is ongoing litigation that will hopefully defeat the handgun roster here in CA since the CA DOJ has made the requirements SO great that manufacturers are not even bothering to submit new models. Which is bad but good for the case moving forward.