The U.S. attorney's office here is investigating charges that Rep. Larry McDonald (D-GA.) used the signatures of dying medical patients on federal firearms registrations forms in a scheme to build a cache of illegal weapons.

McDonald, a urologist by trade, allegedly induced a number of his patients, many of whom were critically ill at the time, to sigh the federal purchase forms so the guns could not be traced to him.

The Treasury Department's Alcohol, Tobacco and Firearms tax unit has joined the probe, looking into allegations that McDonald at one time may have accumulated as many as 200 rifles, handguns and shotguns.

Sources close to the investigation said McDonald devised the scheme before his election to Congress in 1974 from Georgia's industraliazed Seventh District, northwest of Atlanta.

McDonald, 42, is a member of the National Council of the right-wing John Birch Society, and his campaign literature lists him as a member of the National Rifle Association. He receives heavy financial backing for his political campaigns from anti-gun-control groups headquartered in Washington and elsewhere, according to disclosure reports.

The alleged scheme was first revealed by a newspaper here last month. The following day, McDonald released a statement from his Washington office denying he had ever owned 200 guns, but the prepared text failed to address any of the rmaining allegations. He dismissed the story as "a media smear."

However, federal officials say they have been aware of the alleged gun purchase arrangement since late last summer. According to a source here, the case may be within days of going before a federal grand jury.

The alleged scheme is the latest in a series of sensational revelations about the controversial McDonald.

Since he has been in Congress, for instance, he has publicly admitted using the illegal cancer drug laetrile to treat dozens of patients.

Laetrile is banned by the U.S. Food and Drug Administration, which calls the substance a worthless cancer therapy. It is a violation of federal law to ship the substance., which is made from an extract of apriot pits, across state lines.

Also, it was revealed during last fall's election campaign that a close friend and political ally of McDonald's, Marietta physician Dr. Robert Shuman, was treating cancer patients with laetrile and soliciting campaign contributions for McDonald in lieu of taking a fee for his services.

The Laetrile business and the alleged gun purchase scheme may be connected.

Federal investigations have subpoenaed records from two hospitals here McDonald's patients, some of whom were being treated with laetrile. The investigators say they are trying to link the patients signature with gun purchase papers used by McDonald to buy weapons.

In addition, McDonald uses Shuman's Marietta home as a legal residence, in the Seventh District, and that is where the weapons were reportedly stored.

The gun acquisition shceme is an apparent violation of the U.S. Gun Control Act of 1933, which makes it a belony to make knowingly any false statement on federal firearms forms.

McDonald has established a reputation as a maverick since going to Washington three years ago. Just last month, he lost the vote on the House [WORDS ILLEGIBLE] of 410 to 1 when he voted against the establishment of a select 19-member panel named to carry out sweeping ethics reforms.

Although he runs for public office as a Democrat, McDonald showed up recently as "secretary of defense" in the "Shadow Cabinet," some conservative groups recently compiled. That "cabinet opposes the Democrat in the White House from McDonald's state.