California will become the first state to require background checks when buying ammunition after a new law goes into effect statewide Monday.

Proposition 63 backers say requiring these checks for gun sales and not ammunition is dealing with half of the problem. Opponents of the bill say it will hurt law-abiding gun owners, Fox 11 in Los Angeles reported.

Voters approved the measure in 2016. Ammunition sellers said there’s been an uptick in purchases as gun owners try to stock up before the law goes into effect.

CALIFORNIA SEES SURGE IN AMMO SALES AHEAD OF NEW GUN REGULATIONS

“In the last two weeks I’ve been up about 300 percent,” with people “bulking up because of these stupid new laws,” a Sacramento ammunition store owner said.

"Locking a door doesn’t stop a thief from breaking in, so putting another restriction on ammunition it’s not going to stop a crazy person or person with mental illness from getting ammunition or a firearm," an LA-area gun store owner told Fox 11.

A spokeswoman for the Brady Campaign to Prevent Gun Violence said the law is “the kind of thing that could have prevented” the fatal ambush shooting of a 26-year-old Sacramento police officer last week by finding guns that aren’t registered in the state.

The background check will cross reference the gun buyer’s information with that on file at the Department of Justices’ Automated Firearms System that automatically goes on file when someone buys a gun. If the information doesn’t match there will be a small fee and an up-to-10-day waiting period to purchase the ammunition.

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Some opponents of the law are suing the state, saying it violates the 2nd Amendment and is pre-empted by federal law.

The Associated Press contributed to this report.