THE NEW YORK state legislature passed a bill last Friday that Gov. Hugh Carey described as "the toughest handgun law in the country." It is designed to require at least a one-year term in prison for most persons convicted of carrying unlicensed, loaded handguns in public.

That is a drastic -- and welcome -- step in the war against handguns. If it is enforced strictly, and both Gov. Carey and Mayor Ed Koch of New York City claim it will be, one of two things should happen: either the carrying of loaded guns in public will decrease or the New York state prison population will increase dramatically. Last year, more than 4,000 people were arrested in New York City alone for illegally carrying guns. Fewer than 10 percent of them went to jail.

But it was not so much the details of the new law that caught our attention as a description, in The New York Times, of how an old law -- a tough gun-control law already on the books -- is being evaded. The Times persuaded one dealer in illegal traffic to explain how he makes $100,000 or so a year by adding new handguns to the supply in a city where police estimate up to two million such weapons already exist.

It is a tale of hijacking gun shipments (111 such robberies in New York City last year), robbing gun stores (50 to 70 weapons were taken from one store although police arrived no more than five minutes after its alarm sounded) and buying guns legally or illegally in other states. Those acquired outside New York are smuggled across the border in violation of federal law and join the rest being sold quietly on the streets, often to organized gangs. One 17-year-old gang member was quoted as saying, "It's easier for kids to get guns because they don't have 'mans-in-blue' that young." He was talking about undercover officers.

The story makes clear that a single state or city cannot by itself control the handgun menace. New York's new law may discourage some of the gun traffic and gun ownership once word of its terms gets around. And the law may get either some of those who like to tote pistols or their pistols off the streets for a while. That combination could make a dent in the crime statistics (882 persons were murdered with handguns and 33,519 armed felonies were committed in New York City last year). But until Congress enacts stiff federal handgun control legislation and provides the money with which to enforce it, the relationship between handguns, crime and death will be demonstrated daily on police blotters all over the country.