In November of 2016 CA voters passed Prop 63 which would require background checks for ammunition purchases(effective January 2019), make it illegal to buy ammunition online(effective January 2018), and ban the possession of magazines that hold more than 10 rounds(effective July 1st, 2017). As concerning as all of these new regulations are, lets discuss the magazine ban.
Prop 63 states that anyone possessing a high capacity magazine(meaning any firearm magazine holding more than 10 rounds) needs to "get rid of it" by the deadline. People in possession had several options, some of which were turning them over to law enforcement to be destroyed, sending them out of CA to a state where they're legal, and selling them to a licensed firearms dealer. In June 2017 several San Diego county residents along with the California Rifle and Pistol Association filed a lawsuit claiming this new law infringed on their constitutional rights. Federal Judge Roger Benitez temporarily blocked the new ban in his strongly worded 66-page ruling where he expressed concern that this new law would violate firearm owners' 2nd and 4th amendment rights. He stated that unless he issued the injunction that "hundreds of thousands, if not millions, of otherwise law-abiding citizens will have an untenable choice: become an outlaw or dispossess one’s self of lawfully acquired property." This measure of Prop 63 has been suspended until it can be further reviewed and ruled on.
In 2000, magazines holding more than 10 rounds were(with very limited exceptions) made illegal to buy or sell in CA, but not illegal to possess. For over 20 years law-abiding citizens have possessed high capacity handgun magazines, the overwhelming majority of these people have not gone out and committed crimes with these magazines. The reason this ban is considered a violation of law-abiding citizen's rights is because citizens have the "right to bear arms"(2th amendment) to defend themselves and their families and are protected against "unreasonable searches and seizures"(4th amendment) by the government. Prop 63 would essentially take law-abiding citizens and potentially turn them into criminals by forcing them to hand over their property without any compensation.
A criminal, by definition, does not abide by the laws and statutes our government has put in place. So then it is unreasonable to expect that these criminals will follow the same laws that we must abide by. It is illogical to think that, by taking away certain rights from law-abiding citizens the criminals are just going to hand theirs in too, or that they won't continue to be able to still have access to finding these illegal items by unlawful means. It is irrational to think that by putting further limitations on the "good guys" that you'll be in any way restricting the "bad guys". This doesn't make us safer, this instead puts us at an unfair disadvantage when it comes to defending ourselves and our families.
A common argument of those supporting the ban is that high capacity magazines are often used in mass shootings, causing more casualties, and that "common citizens don't need all of those rounds to protect themselves". Who determines that what an individual needs or deems necessary to defend themselves is acceptable? The question has been asked: "why do you need more than 10 rounds at a time anyway?" Well, the question isn't about "need". Why does your car's speedometer say it can go 180mph when you can't legally go over 65mph on most highways? A speeding 2-ton vehicle can cause massive damage and casualties when not used safely and responsibly, should we then regulate all vehicles and cap all speedometers at 80mph over a concern that not all citizens will be safe and responsible with them? After all, look at the statistics of deaths caused by vehicles in crashes, DUIs, and even(more recently) terrorist attacks. But again, the majority of drivers are abiding within the law when out on the roads and it would be unreasonable to enforce strict regulations on lawful citizens because of those who choose to use vehicles recklessly. In the US we have certain rights we can enjoy simply because it is our right to. It is part of what makes this country great. These are things our constitution continues to protect for those of us that chose to abide by our nation's laws.
At the end of the day a law-abiding citizen will still abide by the law and a criminal will still break the law.