"One thing you can say for Jimmy Carter," laughed Phil Kimerer as he concluded a sale of a Ruger 30-06 rifle and 200 rounds of ammunition: "He's good for gun business."

Twenty-four hours after President Carter and Panama's strongman Oman Torrijos signed the new Panama Canal treaties in Washington last week, the tiny Balboa gun shop was crowded with a dozen customers milling around among pistols, rifles and shotguns and examining bullets of all sizes and calibers.

"I heard on the news they'll be knocking down the [Zone] gate near Curundu," a customer accompanies by his wife told Kimerer. "Well, I don't much like it. We live right near there."

In just three hours of business that evening, Kimerer sold "about 20 guns." A few weeks earlier, one night after the agreement was first announced, Kimerer told a friend he sold $5,000 worth of firearms, including a semi-automatic rifle worth $1,750.

Ever since the United States and Panama began to make progress in the treaty talks early this year, the nervousness, sense of isolation and suspicion have spread among many American residents here. Perhaps the clearest manifestation of their fear has been their increased purchase of firearms. Even women who said they never had an interest in guns now go to their local shooting range for target practice. The women's favorites are .38 and .45 revolvers.

Although this mixed civilian and military community of fewer than 6,000 households is dwindling, close to 500 pistols, rifles and shotguns were bought in the Zone in the first six months of 1977, an increase of about 20 per cent over the same period last year. According to Balboa gun-shop manager Kimerer, "Gun sales have shown a good steady growth over the last two years."

The deep-rooted fear of Zone residents, as confirmed in numerous conversations, is that hordes of Panamanians one day will storm into the residential areas of the Zone, ransacking their homes and taking them over. If this occurs, they believe, Canal Zone police and the U.S. military will be outnumbered, or simply will not defend them to avoid a repeat of the 1964 clashes in which 21 Panamanians and three Americans died.

"Everybody I know in the Zone is equipped," said the mother of two children who asked that her name not be used. "Wives are all learning to shoot. We have three guns in the house for protection and we've just ordered a semi-automatic through the gun club. We must protect our own family, you know."

Qualified officials in the Zone appear to have done little to dispel such fears. They are irritated at President Carter and top U.S. officials for using the possibility of guerrilla warfare or sabotage in the Zone as a "selling point" to obtain ratification of the treaty. They call this "dangerous" and more than an invitation for Panama to act." But while they concede that there is little chance of Panama's National Guard's ever invading the Zone, they are afraid of militant political groups' staging a symbolic takeover in a residential area, which could lead to violence.

After a student demonstration in Panama City last week, one well-placed U.S. official sighed with relief: "Thank goodness nothing happened. If they had come into the housing areas in the Zone, I am sure you would have seen civilians using their guns. The cocoon is coming apart here, and people are really afraid."

Whenever Panama stages nationalist rallies, Zone residents and authorities alike react in a way that looks, to a visitor from outside, as though they were under siege. Authorities issue alerts and civilians lock themselves into their homes, turn on police radios and feed one another's nervousness with rumor and alarmist telephone calls.

But over the last year, the Zone residents' sense of vulnerability has been heightened by a sharp rise in crime in the Zone, as Panama economic crisis worsened and unemployment grew. Last year, Zone residents reported 2,021 crimes of theft, including robberies, burglaries and assaults, to Canal Zone police.

Captain Virgil Veyles, the Canal Zone police inspector, described this as a "strong increase, and unusually high for a community our size." Voyles said the police have evidence that most of these crimes are committed by Panamanians, "But we cannot pursue offenders past the boundary line with Panama." On the avenues along the Zone, Voyles added," Robberies have become so bad in the last year, we have put special police teams out. But National Guardsmen in uniform on the other side often stand and watch without doing anything."

Because incidents involving American-owned firearms have been very rare, neither police nor customs officials have kept a record of the firearms in the Zone. "But I believe there is a gun in every house, at least one," estimated Voyles.

Keeping track of the firearms, moreover, is virtually impossible, according to the police inspector, because Zone residents take guns back to the United States, trade from among themselves or sell them to Panamanians who find prices prohibitive in Panama.

With private enterprise forbidden in the U.S. government-run Canal Zone, residents have formed clubs that, as non-profit organizations, may sell imported goods in their shops virtually at factory or wholesale prices.

Of the three gun shops in the Zone, the Balboa store is the largest, and its manager, U.S. Army Capt. Phil Kimerer, opens only on Monday and Thursday from five to eight. Kimerer, who described himself as a gun-lover and competitive shooter, explained that he runs a strict screening system, by selling only to club members who must be U.S. citizens and employees of the U.S. government or dependents of employees. Joining costs $16. Application forms are available in the store and must approved by the gun club board.

Asked about people's motives for buying guns, Kimerer was quick to point out that many of his clients "are sportsmen like myself, and many people buy guns as a hedge against inflation. But it would be naive to assume that many aren't buying arms to protect themselves."

On a land concession of 12 acres, the Balboa gun club operates pistol, rifle and trap-shooting ranges, and Saturday afternoon about a dozen men and women were target practicing.

Some aimed with the obvious ease of near-professional marksmen, while others evidently tried out newly acquired guns. A civilian employee of the U.S. Army, for example, was teaching his wife how to use their new $152 Ruger Security Six.

After she fired a round she said: "I hope I won't have to use it. We've bought it just to keep in the house.We don't know what to expect anymore here."

The accumulation of firearms in the Zone has caused concern among U.S. officials aware that these weapons will become illegal if Panama gains jurisdictions over the Zone.

"The place is a regular arsenal," one official said, "and Panama does not want those guns to stay."

Alfred Graham, the AFL-CIO representative to the negotiations, said he had pressed in vain for a treaty provision for registering firearms or for their transfer to the United States."

"No one wanted to hear of it," Graham said angrily, "they made a big issue in Washington for car, boat and airplane registration, but what are people supposed to do with all those guns?"

Graham, who has lived in the Zone for 30 years, says he fears that "Some people will get hurt before it's all over. There are so many arms around.If the National Guard decides to take the Americans' guns away there will also be trouble, and then I must blame the State Department for part of it."