There is no law prohibiting you from carrying it. Technically the gun was yours the moment you sent off the paperwork for the intrafamilial transfer. It may not be "in the system" yet as registered to you, but as previously stated, a) there is no general requirement for handguns to be registered in California, and b) you may still borrow your parent's gun for a period of up to 30 days at a time, which resets the moment they retake possession of it. The only issues that may arise from carrying an unregistered handgun or one not registered to you, is that there are some sentencing enhancements and misdemeanor -> felony elevators for some crimes. Don't commit any crimes and you shouldn't have to worry. Stay out of school zones and other prohibited places, keep it out in the open at all times, and keep it unloaded. Speedloaders in your pocket are fine.