"Showdown in California" blared the full-page ad in this month's American Rifleman magazine.

A November ballot initiative to register and limit the number of handguns in California, described in the ad as a "forest fire that threatens to rage throughout the country," is the first of its kind. Never before has a gun control measure faced a statewide vote in the nation's most populous state.

Supporters of Proposition 15, as the measure is called, have won endorsements ranging from conservative Reagan "kitchen cabinet" member Justin Dart to liberal political campaign financier Max Palevsky, and raised $700,000 so far for a mass media campaign. Opponents have raised more than $1 million, including at least $565,000 in just one month from the nation's leading gun and ammunition makers.

The National Rifle Association has focused many of its resources on a state that already has a well-publicized handgun ban in San Francisco. "Obviously California is important because it has become a microcosm of what America is steadily becoming," said NRA media relations director Paul C. Stone, who has moved from Washington to California for the duration of the struggle.

Although recent polls show the once substantial lead for the gun control initiative has severely narrowed, initiative opponents are complaining bitterly that television stations have kept some of their advertising off the air. Most television executives have reacted to a 28-minute documentary narrated by Charlton Heston opposing the initiative as if it were the seamiest of X-rated attractions.

Despite the pro-gun group's willingness to pay the several thousand dollars needed to get it on the air, it has not been shown on commercial stations in Los Angeles (station officials say it is too long to qualify under the fairness doctrine) and only in a few scattered places in the rest of the state.

Supporters of Proposition 15 make no secret of their hopes that the measure will have a national impact. By placing the proposition on a state ballot, achieved by gathering 580,000 signatures in less than four months after it was inspired by the attempted assassination of President Reagan, the proponents say they have blunted the influence of lobbying groups like the NRA. In the past, they say, only state legislators and city council members considered gun control measures and they were easily intimidated by NRA threats to campaign for their opponents in the next election.

"We don't have to depend on scared politicians this time," said San Jose police chief Joseph McNamara, one of several big city chiefs who support the measure. "It's the vote of the public."

No state has put a gun control measure on its ballot since 1976, when a proposal to ban handguns in Massachusetts lost by a wide margin, both supporters and opponents say. Howard Gingold, press secretary for the Yes On 15 campaign, shrugged off the Massachusetts failure. He said that proposal had little chance from the start because it called for a costly state purchase of all privately-owned handguns.

In 1978, Gingold said, some gun control proponents tried to get a gun ban on the California ballot but failed to collect enough signatures. "I don't think an outright ban ever would be acceptable," Gingold said. Proposition 15 tries to meet popular objections to handgun bans by prohibiting the legislature from assessing any future ban without voter approval.

Supporters and opponents of Proposition 15 argue heatedly over how cumbersome and expensive it would be to administer, but its elements are relatively simple. It would require all handguns in the state, with some exceptions such as rare guns and target shooting guns, to be registered by Nov. 2, 1983. Anyone carrying an unregistered, concealed handgun in public would be liable for a mandatory six-month jail term. Black market gun profiteers would get a year mandatory sentence.

The measure would put a ceiling on the number of handguns in the state, now estimated at 3.2 million to 5 million, by prohibiting the sale of any unregistered handguns after April 30, 1983, and banning importation of handguns after Nov. 2. Residents could still buy previously registered guns from dealers or from each other, as long as each sale was recorded. This last, unusual feature would make the California law less stringent than the District of Columbia gun control act, which banned the purchase of handguns by D.C. residents after it went into effect in 1977.

The D.C. law has been described by some city police officials as useless because many city residents still easily buy guns in Virginia and Maryland. A study released last year showed the number of handgun murders in the city dropped during the two years after the law passed, but police officials have indicated that handgun slayings have since begun to rise again.

Proposition 15 supporters announced this month a national citizens committee to illustrate the wide-ranging interest in the measure's success. Besides Dart and Palevsky, the committee includes former U.S. treasury secretary W. Michael Blumenthal, former deputy secetary of state Warren Christopher, former housing and urban development secretary and D.C. mayoral candidate Patricia Roberts Harris, former U.S. attorney general Elliot L. Richardson and Elliot Jones, the widow of Washington novelist and internist Dr. Michael Halberstam who was killed with a handgun.

Republican senatorial candidate George Deukmejian, who opposes the gun control measure, has estimated it will cost taxpayers an additional $2 million to administer the registration program, since finger-printing will be necessary to effectively verify the identity of gun owners. Proposition 15 supporters argue that using a driver's license for identification will be sufficient and that administrative costs can be covered easily by a registration fee of less than $10 each.

As for the fear that the ceiling would create a black market for gun runners, Gingold said, "It is not going to be the kind of contraband like dope or heroin. It does not have the resale value. It would take a boxcar of handguns to yield what you get from a pound of cocaine."