As said before single shot exemption requires the gun to be made to receive one round at a time. Shops that do single shot deals will generally revert it back as long as you cross the threshold of the the store. At that point the deal is considered to be done and the shop is no longer responsible for the firearm and its your gun so you turn around and have them revert it back to full function. You will find shops that will not deal with this because they believe it is a gray area of legality and feel they will receive extra scrutiny from the DOJ if word gets around that they do this practice. Even some of the shops that do it like to keep it somewhat low key because you never know who is standing next to you in the shop. It's just one more of the Bay Area wusses trying to make things "safer" for everyone by not having those dangerous guns that fire without magazines all on there own.
My pistols, however, I always kept by me.