A sweeping state gun-control initiative designed to clamp down more on ammunition than rifles or pistols themselves was approved by voters Tuesday.

The idea behind Proposition 63 was to cut gun violence by making it harder to obtain ammunition. Authored by Lt. Gov. Gavin Newsom, the measure will require background checks for anyone selling or buying ammunition, ban ammo magazines of more than 10 rounds, and make it an infraction for dealers not to report bullet thefts to police within 48 hours.

The initiative will also stiffen penalties for those who don’t report stolen guns and intensify tracking of people ineligible to own firearms. It will mandate that California authorities share that information with the FBI for the National Instant Criminal Background Check System, data for which vary from state to state.

The proposition was winning 63 percent to 37 percent with about 40 percent of precincts reporting, a lead that would be nearly impossible to reverse.

Prop. 63 built on a flurry of laws signed this year by Gov. Jerry Brown that were written to make it harder to buy ammunition. Some law enforcement groups that opposed those laws, including the California State Sheriffs’ Association, also opposed Newsom’s initiative. They said the measure wouldn’t stop criminals or terrorists from getting cartridges, would make it harder for law-abiding citizens to arm themselves and could force small ammunition retailers out of business.