If you bought it in CA (records on handguns go back over 50 years) or registered it, they know you have it and a date to go with ownership. The burden of proof is on the proseuction anyway -- which is why cases against long-term residents are rare. When you read about hi-cap arrests it's usually a visitor or new resident.

Originally Posted by
a1bigtuna
I am in Calif. and have a S&W 4006 pistol and bought it and mags for it way before the high cap mag law. The only mags that came with the gun are 12 round and that is all I have. Do I have to carry a receipt that shows I bought it back then?
If you want more detail, legal references, and actual cases go to CalGuns:
http://www.calguns.net/calgunforum/
They can bury you under information about CA law.
People don't like to be meddled with. We tell them what to do, what to think, don't run, don't walk. We're in their homes and in their heads and we haven't the right. We're meddlesome.--River Tam