Koolwrench... with a few exceptions, there is no way to bring a modern off roster handgun into CA from out of state. The few exceptions I mention are pistols listed on the roster of exempted Olympic Competition Pistols (https://oag.ca.gov/sites/all/files/a.../forms/op.pdf?
), or handguns that qualify as a Curio or Relic (C&R).
There used to be what's called an SSE (single shot exemption) for pistols... Sacramento did away with that.
However Single Action REVOLVERS, like a Ruger Blackhawk, Colt SSA or one of clones, are not required to be listed on the roster, so those are legal to bring in from out of state. That also gives one the legal ability to have a non roster double action revolver, sent to a California FFL, who in turn can have a gunsmith convert it to a single action only revolver (on S&W's the DA sear is removed from the hammer), and then it can be transferred (DROS'ed) under the revolver single action exemption to the roster. Once the gun is DROS'ed and picked up after the 10 day waiting period, there is nothing in our laws that prohibits reinstalling the DA sear to restore it back to a DA revolver.
Our gun laws are a patchwork quit of... I can't say what I'm thinking.
To buy an "off roster" handgun like the M&P you want, you need to buy it from a private party, that's close enough for the both of you to do a private party transfer (PPT) at a gun store. I have driven 100+ mile round trips (twice, to so the PPT and then pick it up after the 10 day wait) for S&W's that I wanted THAT badly. Some "motivated" sellers will also be willing to drive to do the transfer.
But be prepared to pay $$$$ for an off roster handgun, they can carry between a $100-$400 premium... I have seen non-shield M&P's being routinely listed in the $900- $1100 range on the CA specific gun forum, and those are the standard versions.
Also, what you experienced is typical for out of state, and even CA gun shops... they do not correctly know the laws, and to make maters worse, these guys that are supposed to know what they're talking about, like the IRS, the answers you get can't be depended upon to 100% accurate either: Bureau of Firearms | State of California - Department of Justice - Office of the Attorney General
I know... I have a C&R FFL, and the stories I could tell you.
Oh... and welcome to the forum!! Feel free to PM me of you have more questions.