California on Monday became the first state to require gun owners to pass instant background checks before buying bullets.

The law requires customers to pay $1 each time they go through a background check to purchase ammunition and $20 if the state's justice department doesn't already have their information in the system.

Democratic California Governor Gavin Newsom and other proponents said the law will save lives. Opponents are suing to undo a law they say will mostly harm millions of law-abiding gun owners.

Voters approved the checks in 2016 and set an effective date of July 1. Ammunition dealers reported a surge in sales as customers stocked up before the requirement took effect.

"In the last two weeks I've been up about 300 percent" with people "bulking up because of these stupid new laws," said Chris Puehse, who owns Foothill Ammo east of Sacramento.

California has 4.5 million registered gun owners. State officials estimate about 3 million are regular shooters and that they will buy ammunition four or five times each year.

The state's justice department, which will administer the background check program, estimates there will be 13.2 million ammunition purchases each year. But 13 million will be by people who already cleared background checks when they bought guns in California, so they are already registered in the state's gun-owners database.

Store clerks will run buyers' identification through that database and a second database of those who bought guns legally but are no longer allowed to own them because of certain criminal convictions or mental health commitments. Those who pass get their ammo on the spot.

Buyers will also have to get their ammo through registered dealers, ending ordering online to be delivered to their doors.

California's requirement follows similar laws in Connecticut, Illinois, Massachusetts and New Jersey. Gun violence declined in those states after they required licenses to buy ammunition, though they also tightened other gun laws, said Ari Freilich, California legislative affairs director for the San Francisco-based Giffords Law Center.

AP