When Sen. Charles E. Grassley (R-Iowa), the gun lobby's biggest trophy of the 1980 campaign, turned his back on his benefactors on a key vote this spring, many people saw it as a sign of the times.

The National Rifle Association, the toughest, orneriest, least compromising lobby in town, seemed to be in the throes of a shooting spree trained mostly at its own feet.

The month before, the NRA had managed to embarrass supporters, amuse enemies and enrage friendlies at the White House by flip-flopping on a favorite whipping boy, the dreaded "gun police" of the Bureau of Alcohol, Tobacco and Firearms.

Afer trying for years to bury the bureau, the NRA lobby wound up scrambling to rescue it lest a more credible agency, the Secret Service, take over enforcement of gun laws.

About the same time, NRA was failing in court and in a state legislature to undo the nation's first outright ban on handgun possession, passed last year by the Chicago suburb of Morton Grove, Ill.

After Morton Grove came more trouble. The gun-control movement, stymied at the federal level by the NRA's fabled clout here, began erupting across the land. Last spring, Chicago passed a tough new handgun freeze, and last week San Francisco enacted an even tougher, modified pistol ban.

In what may be the most ominous development of all, gun-control forces in California collected enough signatures to place a referendum on the November ballot that would cap handgun possession at its current level in the nation's most populous state.

"This is our breakthrough opportunity . . . our chance to pierce the myth of the gun lobby's invincibility," said John Phillips, director of Californians Against Street Crime, which will spend almost $2 million on a sophisticated advertising campaign to promote what it hopes will become the first statewide handgun cap. Early polls show the proposition ahead.

It is hardly surprising to find a bit of bunker mentality settling in at the NRA's austere marble and granite national headquarters building just south of Scott Circle NW.

"Gun owners are the new niggers . . . of society," said John Aquilino, the NRA's official spokesman. Those are strong words, but Aquilino can at least be thankful that he has a job.

Three of his superiors were summarily fired this spring in one of the periodic shake-ups that always seem to be rocking the organization. One of the scalps belonged to the NRA's hard-driving chief lobbyist, Neal Knox, a crack marksman and near legendary figure in gun circles.

Knox and the man who fired him, NRA executive vice president Harlon Carter, had been key figures in engineering the famous "Cincinnati Revolution" of 1977, which purged suspected "soft-liners" and conservationists from the NRA leadership and transformed the organization into a modern, computerized, politically sophisticated lobbying juggernaut.

Now, the king has jettisoned the prince because, some said, the prince was coveting the throne. Even for as intrigue-ridden an institution as the NRA, the housecleaning was a shocker. In the absence of any official explanation, tea-leaf readers appeared in force.

"The NRA is a dinosaur slowly strangling itself," theorized Michael Beard, director of the National Coalition to Ban Handguns, one of several gun-control groups formed in the late 1970s to combat the gun lobby. "When that starts to happen, the internal battles usually get more intense."

But are things really all that grim? Is the gun lobby losing its bang? And is the balance of political power on the gun issue beginning to shift toward control after a decade and a half of uneasy stalemate?

Answers to these complicated questions are more murky than the evidence of the last few months might suggest.

Nothing in NRA's balance sheets or membership rolls betrays the slightest hint of organizational fatigue. For NRA, the bad times have never been so good.

The organization has 2.4 million members, more than double the total three years ago. It has a $4 million lobbying budget, 350 employes, $40 million a year in revenue from membership dues and from gun ads in its enormously popular magazines, American Hunter and American Rifleman, and an investment portfolio of more than $40 million.

Its political action committee will raise almost $2 million this year to help sympathetic candidates, and its special fund-raising appeals to underwrite opposition to the California referendum campaign are expected to bring in another $5 million.

Moreover, it is signing up new members at the riveting clip of 44,000 each month. By contrast, membership in Beard's handgun-control group, after a surge in new faces and contributions following the shootings of John Lennon and President Reagan, is close to pre-1981 levels. "We went from a three-phone shop up to a 10 phone-shop and now back to a four-phone shop," Beard said.

The NRA has long known how sweet are the uses of adversity. Its line on gun control has always been that the other side actually favors confiscation and bans, not the mere sugar-coating of registration and control. Once your pistols are taken, will your deer rifles be safe? the NRA asks.

To those who accuse the group of demagoguery, of manufacturing threats so it can cynically ring alarm bells, the NRA finds a measure of vindication in places such as Morton Grove where, although the issue has stayed with handguns, it has moved up a notch from control to bans.

"The nice thing about Morton Grove is that the people who have been beating their breasts about moderate controls have been exposed for the frauds they are," Aquilino said.

The gathering force of the gun banners has produced a backlash in Kennesaw, Ga., and six other small communities throughout the country that have enacted laws requiring each household to own a firearm.

While few expect such laws to take root, there has been a phenomenal increase over the last two decades in ownership of firearms in this country.

Although a Gallup poll shows that 66 percent of Americans favor stiffer gun controls and 41 percent favor bans, the marketplace shows that Americans are in favor of lots and lots of guns. An estimated 150 million to 200 million firearms are in private hands in this country, and one of every two households has a firearm of some kind.

The number of handguns, the subject of most of the gun-control debate, is estimated to have tripled in the last two decades to 50 million. Small wonder that in 1979, there were 48 handgun murders in Japan, eight in Great Britain, 34 in Switzerland, 52 in Canada, 58 in Israel, 21 in Sweden, 42 in West Germany and 10,728 in the United States.

For decades, sociologists and psychologists have said guns are a particularly American obsession, protected by the Bill of Rights, evocative of the nation's pioneering spirit and somehow bound up in the American rite of passage into manhood.

But more recently, in a era ridden with violent crime, they have become the option of choice for those thinking self-defense--urban and rural, women and men, elite and common. In these new gun purchasers, the NRA sees a vast constituency that will beat back gun banners at the grass roots.

"We're going to win in California," said J. Warren Cassidy, who replaced Knox as NRA's chief lobbyist. Cassidy, 51, a Dartmouth graduate and former insurance executive, earned his gun lobby spurs leading opposition to the only other statewide referendum on guns, a 1976 proposition to ban handguns in Massachusetts. It was resoundingly defeated by 69 percent of the voters.

The California measure is considerably less stringent--"We drafted a measure we thought we could pass," Phillips said--calling for a cap rather than a ban. Still, there are an estimated 5 million gun owners in California, proportionally more than there were in Massachusetts. "You wouldn't want to run Hollywood through a metal detector," Aquilino noted.

To the gun lobby, new gun buyers are "voting with their feet" on gun control. Their very numbers, the gun lobby says, give the lie to the way advocates of control and their media sympathizers have formulated the gun issue. That formulation goes something like this:

* Most Americans favor stiffer controls, but the minority who oppose them are more passionate, better organized and have found a way to thwart the majority will.

* They have discovered that by controlling just 2, 3 or 4 percent of the votes in a given political campaign, often the margin of victory or defeat, they can exert influence far beyond their numbers. Candidates who fear their voting bloc or covet their money will sign questionnaires their way.

* As a consequence, there is the current paradox. Even as communities adopt stiffer gun-control laws, the momentum goes entirely the other way on Capitol Hill.

For the first time in two decades, the gun lobby is on offense in Washington. The McClure-Volkmer bill, drafted by the lobby, has 61 cosponsors in the Senate and 172 in the House and would gut the most important provision of the landmark 1968 Gun Control Act by removing restrictions against sale of firearms by mail. The bill is due for a Senate vote this summer.

But as the NRA is on the advance on Capitol Hill, another paradox has emerged: it finds itself colliding with roadblocks and resentments it never confronted as a basically defensive lobby.

Even friends such as Grassley and Sen. Robert J. Dole (R-Kan.) stray from the fold occasionally, perhaps in part because they resent the lobby's heavy-handedness.

Dole said he had his fill of the gun lobby during his brief 1980 presidential campaign.

"I resented . . . that sort of quizzing in public, trying to lock us into every little detail that the National Rifle Association might promote. . . . You have to have a litmus test every five minutes or you're considered wavering. . . . The whole thing is sort of crazy."

The NRA's new chief lobbyist acknowledges a problem and wants to smooth the hard edges.

"Politics is the art of the possible," said Cassidy, who learned his as a one-term mayor of Lynn, Mass. "There have been lobbyists at the NRA whose zeal has occasionally gotten in the way of their common sense."

He cited his unhappiness about how war against the BATF was waged. "I think we went on a little bit of an ego trip; we tried to flex our muscles to show how powerful we were," he said.

Is NRA ready to strike a more compromising legislative posture? Or is the new congeniality mere window-dressing? There can be no compromise on the basic question of controls, said Cassidy, who reportedly fears that Knox may be masterminding a counter-coup at the next NRA board meeting.

The gun lobby of the last five years has never been known to yield an inch, whether the issue is a computerized registration system to trace guns used in crimes, a "Keep Away From Children" label on explosive cartridges or a proposed restriction on sale of armor-piercing "cop killer" bullets. It is the captive of its own hard-line rhetoric. In its lexicon, any new regulation is another step on the slippery slope toward confiscation.

The Grassley vote is a case in point. In the Judiciary Committee this spring, Grassley voted for an amendment to McClure-Volkmer, drafted by Dole and Sen. Edward M. Kennedy (D-Mass.), that would require a 14-day waiting period between buying a gun and picking it up.

That period is designed to cut back on crimes of passion, and to give law enforcment agencies a chance to investigate criminal and mental health records of gun buyers.

Some lobbies might quietly view such an amendment as just the sort of sop to the opposition that would ensure passage of an overwhelmingly favorable bill. Said handgun-banner Beard of the waiting period: "It's like a lifeboat on the Titanic. The bill is still a disaster."

But the NRA is vowing to rip the amendment off the bill on the Senate floor, thereby creating problems for itself in the less sympathetic House, and extract a measure of retribution against those who voted for it. Grassley, whose campaign received $56,950 from the gun lobby in 1980, will find himself in the hot seat.

"I plan to make a call on Sen. Grassley soon and try to persuade him to change his vote on the floor," Cassidy said. "If that fails, then that becomes a part of the record. We would have to try to replace him with a more reliable voice."

Just as quickly as the threats tumble out of his month, Cassidy flashes his other side, his politician/salesman, saying he does not believe that over the long haul the NRA should punish its enemies. "I'm not so fatheaded that I think the average senator is quaking in his boots at me," he said.

Cassidy's long-term answer is what he calls a "soft sell" to the nation. He wants to "show off the good side of our lobby" through the Boy Scouts, Junior Chamber of Commerce and in-school programs.

"Maybe not in my generation but maybe in my children's, people who don't own guns will have been brought up with a more reasonable view of what they are all about," he said.

The NRA and Cassidy are confident that they have one more hole card.

"The reason we will never lose is that the most basic human drive is for survival. Not power, not sex--survival.

"We live in an age of more and more crime, less and less punishment, and there is no way the limousine liberals . . . are going to tell the people they cannot have the means to defend themselves. It just won't fly."