What do Ruby Fox, Hugh McGuigan and Mark David Chapman have in common? Answer: All are prominent customers in one of the United States' oldest established -- and most controversial -- consumer markets: the $235 million retail handgun industry.

Fox, of Parker, Ariz., is an avid marksman -- holder of the National Rifle Association women's outdoor pistol-shooting title. McGuigan is a California highway patrol sergeant. And Chapman is being charged in last week's shooting of rock star John Lennon.

These three disparate -- and admittedly unrelated -- examples serve as vivid illustrations of both the scope and the problems of the nation's growing handgun-manufacturing and -importing industry:

The spate of recent shootings -- Lennon in New York City, cardiologist Michael J. Halberstam in Washington and diet doctor Herman Tarnower in Scarsdale -- have revived public interest in the handgun market.

At the same time, handgun-manufacturing is one of the most fiercely competitive, least regulated -- and most secretive -- industries in the United States. It's also one of the most aggressive politically, protected by a well-financed and highly effective lobby.

Who makes and sells the millions of pistols and revolvers that are marketed in the United States each year, and how do they get into the hands of people like Mark David Chapman? How profitable is handgun-manufacturing? And why does the industry play things so close to its chest?

The answers are difficult to obtain, in part because there are so few statistics available on handgun production and marketing. The industry itself is tight-lipped about disclosing figures. And the few government tabulations provide little real information about how the industry works.

However, from excise-tax figures collected by the Treasury's Bureau of Alcohol, Tobacco and Firearms and studies by Henry S. Bashkin, a former Commerce Department economist now serving as research director of Handgun Control Inc., this picture emerges:

The handgun market in the United States is approaching -- and will fast exceed -- a quarter of a billion dollars a year, making pistol- and revolver-manufacturing an important, albeit relatively modest, sector of the economy. Last year there were 2.17 million handguns produced here at home. Imports totaled 231,000.

The industry is comfortably profitable, but returns vary widely depending on the firms and the types of weapons produced. As in autos, a handful of big companies dominate the domestic industry. However, competition often is fierce. And for many firms, handgun production is only a portion of their business.

Unlike many sectors of the economy, the industry is virtually free of government regulation -- even involving routine standards for firearms safety. Moreover, critics charge that federal regulators too often have been lax -- overly sympathetic or simply too cozy with the industry.

Perhaps more than any other domestic business, the industry is shrouded in secrecy -- and adamant in refusing even to talk about its own structure and economic condition. Politically active, it finances a powerful and well-heeled lobby, which has been successful in fending off efforts at regulation.

By most standard measures, the U.S. handgun market has remained relatively stable in recent years. Besides the 2.17 million handguns that domestic firms produced in 1979, the United States imported some 231,000 foreign-made units. Exports of U.S. handguns overseas totaled 224,000.

Handguns include pistols and revolvers, not shotguns or rifles. Besides police work and crime, handguns are used for target-shooting, collecting and "plinking" -- backyard firing at tin-cans and bottles, an entirely separate sport as far as gun-owners are concerned.

Although there aren't any estimates of the market's size, Bashkin figures -- based on Treasury reports of gun-related excise-tax collections -- that dollar volume at wholesale runs to $175 million, pointing to a total of $235 million at retail last year.

However, that doesn't include either the imports -- which added another $21.3 million at wholesale last year -- the guns manufactured here from foreign-made parts or the guns sold illegally or on the black market. How much are they worth? Says Bashkin: "It's anybody's guess."

The domination of the handgun industry by a few giant firms is almost as apparent as in the auto industry. Figures extrapolated from 1977 tax receipts show three major manufacturers -- Smith & Wesson, Sturm, Ruger & Co. and Colt Industries -- produced 70.3 percent of U.S.-made handguns that year.

The rest comes from a few dozen small manufactureres.

The weapons are marketed through a complex, and often not well-organized, network of distributors and local dealers, any of which may buy directly from the manufacturer.

Industry experts estimate there now are 150,000 dealers, including pawnbrokers and other part-timers. There's also a healthy black market -- dealing in both stolen guns and in large-volume sales in regulation-free states.

Perhaps the fastest-growing portion of the industry, however, involves imports.

Technically, the importation of cheap, easily-concealed handguns is forbidden by the 1968 Gun Control Act. And, indeed, overall handgun imports have gone down, to 231,600 in 1979 from a peak of 747,000 in 1967.

However, as federal enforcement officers readily concede, foreign manufacturers have discovered they can get around these prohibitions easily by shipping only the parts, which then are assembled in U.S. plants.

As a result, Commerce Department figures show there's been a dramatic increase in the volume of handgun parts imported into the United States in recent years -- to $5.36 million worth in 1979, up from $1.8 million in 1976.

What's more, that traffic has been spurred, inadvertently, by the new Multilateral Trade Agreement the United States ratified earlier this year in which the White House slashed tariffs on imported handgun parts by up to 60 pecent.

Although President Carter asserted repeatedly he favored more stringent handgun control, U.S. trade negotiators agreed to reduce the maximum duty on foreign handgun parts to 8.2 percent from 21 percent.

Of the 1976 total, Bashkin's computations show $1.1 million worth was shipped to Miami alone. Other totals then included New York, $368,000; St. Louis, $146,000; and Baltimore, $67,000. Washington bought $58,000 worth.

The privately financed organization reports that a spate of new assembly plants have sprung up in Florida and other regulation-free southern states over the past several years.

What's more, Bashkin's own research -- confirmed privately by ATF -- shows that a large proportion of the small, inexpensive handguns manufactured by domestic firearms makers couldn't be imported under current restrictions on purchases of foreign-made Saturday-night specials.

The industry's profit picture is difficult to draw a bead on. Company sources confirm that manufacturers' markups average between 25 percent and 30 percent, with profit margins in the 7 percent range.

A handgun that is list-priced at, say, $175, will cost the local dealer about $150. The distributor will have paid the manufacturer about $125 or $130 for it. It costs the manufacturer about $100 to make.

Industry officials refuse flatly to discuss which lines are most profitable for individual companies -- particularly the Saturday-night specials, which are cheap but sold in high volumes by some firms.

However, Bashkin points out that the gun manufacturers operate in a relatively rarified business climate: Most schedule production a full year in advance, so inventories always are low. Shipments invariably equal production.

List prices of U.S.-made pistols and revolvers in the Saturday-night-special category in 1977 ranged from a minimum of $33.50 for an Excam Inc. Targa G-27 to a high of $300 for a Four-Ace 300.

And those prices are only for new guns. Many used weapons, and some sold on the black-market, range even lower.

One factor bolstering the financial health of many major gun makers is that their firearms business is only a portion of their overall operation. R G Industries, for example, manufacturers a variety of other sporting goods. And Colt, Smith & Wesson, and Remington are parts of giant conglomerates.

Harry Hampton, executive director of the Sporting Arms and Ammunition Manufacterers' Institute, the industry's trade group, insists that handgun-manufacturing offers companies "the lowest returns on any recreational products" made in the United States.

However, he concedes, "there are a lot of small companies each year that want to get into the handgun business, although it's not that easy to get into production."

For all the hullabaloo over new laws and restrictions, the handgun industry, remains one of the least-regulated in U.S. business.

While an army of government agencies regularly sets standards for and polices the manufacture of other, far-less-dangerous equipment, there are no federal standards or requirements for firearms safety.

Although gun manufacturers legally must be licensed, anyone can qualify by paying a $50 Treasury fee. No background check is required. Dealers must pay $10 for their permits -- also on a no-questions-asked basis.

And, while gun retailers must keep records on whom they sell their merchandise to, they're not required to verify the accuracy of an applicant's claims as, say, pharmacists must do on prescription drugs.

yimportantly to some critics, there also are no security requirements for storage of these guns -- neither by manufacturers, distributors nor local dealers. As a result, pilferage and theft are easy.

Smith & Wesson, a Bangor Punta subsidiary, endorsed licensing of handguns in 1976 and became the target of massive industrywide retaliation. Smith & Wesson since has softened its stand.

The United States is alone among the major industrial nations in not restricting the use of handguns. Canada, for example, prohibits handgun sales to all but members of the armed forces and police. Other countries have even more stringent regulations.

There also have been charges that ATF, as the government's top firearms enforcement agency, has been too sympathetic to, and cozy with, the industry.

Is the recent raft of shootings giving the gun manufacturers second thoughts? Certainly not, they unanimously agree.

Says Guenter W. Bachmann, a Smith & Wesson vice president: "This is a logical and an honorable business activity. There's no need to add any more regulations."

In the meantime, the Uniteds States' growing handgun industry is very much alive and apparently well. The pistols, revolvers and automatics are available for sale any time that Ruby Fox, Sergeant McGuigan -- and Mark David Chapman -- want them.