The summary of the proposition as it appeared on the general election ballot: Requires background check and Department of Justice authorization to purchase ammunition. Prohibits possession of large–capacity ammunition magazines. Establishes procedures for enforcing laws prohibiting firearm possession by specified persons. Requires Department of Justice's participation in federal National Instant Criminal Background Check System. Fiscal Impact: Increased state and local court and law enforcement costs, potentially in the tens of millions of dollars annually, related to a new court process for removing firearms from prohibited persons after they are convicted.
Information on the language of the proposition is from the Official Voter Information Guide issued by the California Secretary of State: http://vig.cdn.sos.ca.gov/2016/general/en/pdf/complete-vig.pdf
Source for Popular Vote data: New York Times. California Proposition 63 - Backgrounds Checks for Ammunition Purchases and Large-Capacity Ammunition Magazine Ban Initiative, New York Times (https://www.nytimes.com/elections/results/california-ballot-measure-63-firearm-and-ammo-control) (accessed 11 Dec 2017)