Price Is About $800

It is also possible, for about $800, to purchase a semiautomatic MAC-10, a kit to convert it into an automatic and the key components of a silencer in less than 45 minutes with no questions asked.

Other semiautomatic weapons can also be converted to automatic fire, but it is particularly easy with the MAC-10, according to Federal authorities. One model of the semiautomatic MAC-10 is so easy to convert it is legally classified as a machine gun, even though it only fires one shot at a time.

The weapons can be obtained in a number of ways. Private individuals are permitted under Federal law to make ''occasional'' sales of weapons, including semiautomatics. Federal law-enforcement officials say many semiautomatic weapons change hands in private, unregulated sales that leave no paper trail.

''For all legal purposes,'' Gary Peach, chief of the enforcement group of the Federal Bureau of Alcohol, Tobacco and Firearms in Jackson, Miss., said of the MAC-10, ''it's a .45 pistol,'' and the parts to convert it are ''basically machine parts.'

Modifications Are Needed

With the conversion kit, the weapon can be modified at a machine shop or a well-equipped home workshop to function as a submachine gun. A modified weapon, with or without the parts from the conversion kit installed, is considered an automatic weapon under the law.

In the same way, someone who owns all the parts of a silencer, assembled or unassembled, is violating the law prohibiting the possession of a silencer. It is legal for a dealer to sell some of the parts to a silencer, but not all of them.

Semiautomatics are offered for sale at several hundred gun shows or fairs held around the country each year. In a recent gun show in Missippi, a number of semiautomatic weapons, including at least a half dozen MAC's, were offered for sale. A man who said he was a private collector from Meridian, Miss, sold one for $600 in cash and the buyer was not asked to produce identification. A conversion kit was available at a nearby booth for $29.95. That booth also sold the internal components of a MAC silencer for $90. A few yards away the remaining silencer part, a container tube, sold for $39.95.