The House Assassination Committee yesterday disclosed that sophisticated new tests have matched fragments of metal from the wrist of former Texas governor John Connally with the so-called "magic bullet" from Lee Harvey Oswald'a Mannlicher-Carcane rifle.

The linkup of the bits of metal from Connally with the nearly intact bullet found on his stretcher at Parkland Hospital 15 years ago was made during neutron activation tests conducted last fall at the University of California at Irvine.

A panel of firearms experts confirmed in separate tests at the metropolitan police firearms laboratory here that the bullet from Conally's stretcher-known to assassination buffs as CE (commission exhibit) 399 - came from the 6.5 mm. Mannlicher found in the Texas School Book Depository after President Kennedy's assassination.

In a compelling sort of scientific detective story that unfolded in a largely deserted hearing room. Dr. Vincent P. Guinn, a University of California professor of radio-chemistry, told the committee that the tests he conducted also left little doubt that fragments of metal from Kennedy's brain, as well as all other fragments turned over to him, came from another Mannlicher-Carcano bullet.

He said there was no evidence that any other bullets struck the presidential limousine in Dallas on Nov. 22, 1963.

Ironically, Guinn told the committee, the FBI had much the same data from earlier, more primitive neutron activation tests in 1964, but evidently didn't know how to read the results.

The Warren Commission kept the fact that the earlier neutron tests had even been conducted a closely held secret. Critics charged that the results had been suppressed because of fears that they might upset the commission's "single-bullet" theory: that one shot wounded both Kennedy and Conally almost simultaneously before the president was hit by another, fatal bullet.

Guinn said the FBI data, finally released in 1973 as the result of litigation under the Freedom of Information Act, baffled him at first, too - until after he conducted his own experiments as a committee consultant.

"I initially could not make any sense of it," Guinn said of the FBI data. But with the "hindsight" of his own tests, he reviewed all the old, jumbled numbers again and found they jibed substantially with his own findings.

"I was frankly surprised to see even their data fell right into the same picture," he told the committee.

Neuron activation tests are experiments carried out with the help of a nuclear reactor and sophisticated detection equipment to determine whether samples of various materials, such as bullets lead, might have a common origin.

Guinn, a neutron testing expert who has testified in dozens of court cases, said he was helped by the fact that the Mannlicher-Carcano ammunition made in this country had a much lower antimony content than most bullets. It also varies widely, from one Mannlicher-Carcano cartridge to the next, even within the same box of 20.

"This is not true of most of te bullet leads we'd ever looked at before," Guinn told the committee. He said his tests showed that the "pristine bullet" from Connally's stretcher had traces of antimony and other elements, such as silver, strikingly akin to the fragments doctors had taken from Connally's wrist.

Similarly, Guinn testified, a large mashed bullet fragment found in the front seat of the president's limousine and smaller fragments found on the rear floor were clearly like three fragments he tested that had been taken from the president's brain.

Pressed by Rep. Floyd J. Fithian (D-Ind.) about how certain his findings were, Guinn replied with scientific reserve.

"I would say highly probable," he told the committee. "I wouldn't want to say how high-whether 90 or 90 percent or 99.4."

At the committee's behest, Guinn also tested the bullet retrieved from the home of retired Army Gen. Edwin Walker, whom Oswald had alledgedly tried to shoot in April 1963. He said it was "extremely likely" that the slug was from a Mann licher-Carcano.

Guinn's tests also created a new mystery, however. The fragments the FBI tested in 1964, he told Fithian, have all disappeared. Guinn said he carefully weighed the bits and pieces of metal brought out to him by officials of the National Archives last year and not one of them matched the fragments recorded in he FBI data.

"The pieces brought out by Archives did not include any of the specific pieces the FBI analyzed," he testified. "Where they are, I have no idea."

Elaborating to reporters later, Guinn said, for example, that he was presented a small container ostensibly carrying all the bullet fragments from Kennedy's brain. It contained two bits of metal, one weighing 41.9 milligrams and the other 5.4 milligrams. yet, Guinn said, the FBI records showed four other samples from Kennedy's brain, all with diffeerent weights.

In the same fashion, the FBI data indicated that it had tested three bits of metal from Connally's wrist at Oak Ridge National Laboratories in 1964, two weighing 2.3 milligrams each and another weighing 1.52 milligrams. The container Guinns got, which he said came with assurances from Archives that this was all the metal from Connally's wrist in his possession, and two other pieces, one weighing14.4 millgrams and the other 1.3 milligrams.