Don't panic. It is in many ways not quite as bad as you fear, though in some ways it's worse...
The governor of CA already signed into law a raft of restrictive measures an few months ago, this measure will alter the effect of those measures.
Just to sum up a few points of the *existing* law:
The ammunition restriction doesn't start right away, with stages coming in at the start of 2018 but doesn't take full effect until the middle of that year. Prop 63 will accelerate this process by having the whole thing in place by the beginning of 2018.
It will be illegal to bring more than 50 rounds of ammunition across the border without proper paperwork. You can still sell ammunition to a friend, same 50 round limit.
As to reloading components, I understand why people are confused, section 16150 has two definitions of ammunition. One is only a loaded round and the other is anything that even resembles gun-type stuff. Only the first is subject to the background check, the second is a prohibition on selling anything gun-related to anyone you know-or suspect to be shady. It's pretty slippery-slope but it shouldn't stop reloading components from being mail-order.
I myself think people are glossing over a few of the more disturbing parts of the law. I do wish people would read laws before commenting on what may or may not come to pass so here you go:
California Proposition 63 The Safety for All Act of 2016 - The Voter's Self Defense System - Vote Smart
or the PDF
http://smartgunlaws.org/wp-content/uploads/2015/10/SafetyForAllActFinal.pdf