As Gualberto Jimenez returned home from work the other day, he found a strange car in his driveway and noticed his front door had been forced open.

Suspicions aroused, police later reported, Jimenez pulled out a 9 mm semi-automatic pistol, confronted the apparent burglar and fatally shot him when the intruder moved his hand toward a pocket.

The deadly drama was one of many in this south Florida city, where guns play a starring role. Lax firearms laws, big-time drug dealers who enforce their own violent justice and proximity to gun-hungry Latin American nations have made Miami what federal officials consider the nation's most active gun market.

The availability of weaponry of all types increases the danger faced by local police and task force officials engaged in a major campaign to reduce the flood of illegal drugs entering the United States through the Miami area.

Customers in local gun shops can browse among arms ranging from sawed-off shotguns through an array of easily concealable handguns to well-known submachine guns such as the Israeli-designed Uzi and the 9 mm MAC10. The Garcia Gunshop in the Miami district of Hialeah was featuring a special on the Uzi this week: the blunt little killer with a long, thin magazine was for sale at $525.

Under federal law, fully automatic weapons such as the MAC10 or the Uzi must be registered on sale and are subject to a stiff tax. As a result, gun dealers modify the weapons to make them semi-automatic. This requires that the trigger be pulled for each shot, but the weapons easily can be modified again to make them fully automatic: one pull of the trigger for a continuous burst of fire.

"In about three minutes, if you're slow, you can convert this to fully automatic," said an agent of the Treasury Department's Bureau of Alcohol, Tobacco and Firearms, gesturing with a MAC10.

Modified to semi-automatic, the weapons can be purchased like any other gun in Florida. A customer fills out a form in which he declares he is over 18, is sane and does not fall in the category prohibited by law from owning weapons (such as parolees). The form remains on file with the gun shop and the customer walks out with the weapon.

In Dade County, which includes Miami, regulations force a 72-hour "cooling-off period" before customers can leave with a handgun. This does not apply to long guns, however, as Carl Brown found out last month.

Brown, 51, a teacher, reportedly bought a 12-gauge riot-type shotgun on Aug. 19 at Garcia's, and the next day killed eight persons at a Miami repair shop. As he tried to escape on a bicycle, Brown was shot dead by a passerby with a pistol, who later received a commendation from a Miami citizens' group.

No charges have been filed against that passerby or against Jimenez.

In Broward County, which includes Fort Lauderdale, the growing megalopolis between there and Miami, gun buyers encounter no waiting period even for handguns. As a result, Treasury agents say, many Dade County residents drive a few minutes up the freeway to make their purchases.

"Broward is O.K. Corral," said Charles Intriago, a Miami lawyer who heads the Florida Coalition to Halt Handgun Crime.

Intriago and his group are seeking restrictions on handgun sales here because, they say, the availability of such weapons contributes to violent crime that has led to more than 400 homicides in Dade County so far this year. The District of Columbia so far this year has had 162 homicides. Its population is 638,432, in comparison with the population of Dade County, which is 1,625,781.

In Florida, police officers were assaulted by firearms 451 times last year, up from 345 in 1980 and 251 in 1979, according to the Florida Department of Public Safety.

But many -- perhaps most -- of the weapons are purchased to be smuggled to Latin America for resale, federal officials report. Because of false identities and lies on required forms, combined with lack of manpower to check them all, agents are unable to say with precision how many weapons end up in the hands of Florida residents and how many travel north to New York or south to Central and South America.

A measure, however, comes from the federal requirement that gun dealers report to the Bureau of Alcohol, Tobacco and Firearms every time a single buyer purchases two or more handguns within five days. The Miami BATF office receives from 600 to 800 such reports a month, one agent said, adding:

"We know most of those firearms are going out of the country, and that most of them are going to Central and South America."

Agents tailed one Latin American who landed at Miami International Airport, took a taxi to Broward County and, with the meter running, purchased two suitcases full of pistols and then rode back to the airport for a flight home. He never made it, however, because agents arrested him on charges of violating the Neutrality Act that requires a State Department permit to export weapons.

U.S. customs officials say Miami is the busiest outlet for gun smuggling to Latin America because of geography and ties created by south Florida's drug traffic with Latin countries. Outgoing shipments have been discovered in chemical drums, washing machines, plumbing fixtures and, on one occasion, in suitcases so stuffed they ripped open at the airport.

Other weapons go out in boats and planes. Sometimes they travel in the same craft that transported marijuana or cocaine north from the Latin countries that in turn will be the market for the southbound guns.

"It's a very good commodity to trade for the narcotics," a customs investigator said.

Like other federal officials interviewed, he requested anonymity.

Profit is high for a gun smuggler who makes a successful run. Customs agents estimated that a 9 mm Browning pistol bought here for $450 sells for about $2,000 in Caracas.

Treasury agents report the profit also is high on above-board deals, so high that two Hialeah policemen, A. Edward Preston and Thomas Nevins Jr., applied last month for Treasury licenses to produce a light-weight submachine gun similar to the Uzi in a local warehouse. The application is pending.

Most traffic, officials believe, revolves around small weapons bought here by entrepreneurs and sold for profit in Latin America to private citizens seeking protection from kidnaping or assault in turbulent political conditions, or to drug smugglers trying to protect themselves from local police.

Some traffic, however, involves larger-scale weaponry for political groups. Bernard Sansaricq and six of his followers accepted a no-contest guilty judgment in U.S. District Court here last month on Neutrality Act charges stemming from their capture at sea last January with several dozen weapons designed to help what customs officials said was a coup attempt against President Jean Claude Duvalier of Haiti.

To reduce the city's role as an arms center, Intriago's group has proposed that the Broward County require handgun registration and Treasury agents have obtained a court ruling against further manufacture of the easily convertible MAC10 submachine guns.

But at the same time, the Florida legislature in this year's session passed a law abolishing the rule that handguns in automobiles must be secured in the glove box. A simple holster now suffices.

"We're more on our toes than anyone else in the country," said Dade County Policeman Tim Davis.