SOMERVILLE, N.J. (AP) Riddled with worn parts and debris, Jayson Williams' shotgun was ``an accident waiting to happen,'' a weapons expert for the retired NBA star testified at his manslaughter trial.

Although the judge told the jury to disregard that comment, much of the testimony gave support to the defense claim that Williams accidentally shot and killed a van driver after snapping shut the shotgun. The defense also maintains the model was prone to malfunction.

The expert, Richard N. Ernest, said the firing mechanism was worn and contained wood chips, rust and oil, making it able to discharge without the trigger being pulled.

With the aid of enlarged photos projected on two screens, Ernest gave the jury a course on the tiny pieces that link the trigger to the firing pin in Williams' 1993 Browning Citori 12-gauge double-barreled shotgun. He noted locations where metal had worn away due to use, and described the small dimensions involved.

For instance, he pointed to the area in which two of those components, the hammer and the sear, meet. In a new Citori, the hammer is restrained from striking the firing pin by a ledge of metal on the sear that measures only 14 to 16 thousandths of an inch _ the width of three sheets of typing paper _ Ernest said.

But the wear on Williams' gun has reduced the contact area of the metal, he said: ``These surfaces are not going to be able to mate together properly, and they are hanging by a thread.''

``The more rounded these surfaces become, the easier it is for the weapon to malfunction,'' said Ernest, a former crime scene investigator with the Georgia Bureau of Investigation who now owns Alliance Forensic Laboratory in Fort Worth, Texas.

State Police Detective Sgt. James Ryan, the prosecution expert who tested Williams' gun, has testified that the weapon operated properly, firing only when the trigger was pressed and the safety was off.

But Ernest raised questions about Ryan's competence, saying that Ryan was unaware of testing procedures in place for two decades, and suggested the officer did not know how to disassemble the weapon.

In addition, he said a shell in the lower barrel of the shotgun cannot be seen even by the holder of the weapon when it is cracked three-quarters open. That cast doubt on the credibility of a prosecution witness, former Williams teammate Benoit Benjamin, who said he was standing several feet from Williams and saw a shell in the lower barrel.

Ernest is to continue testifying when the trial resumes Monday. He is expected to describe how he induced misfires in a similar Browning Citori as that used by Williams.

Over prosecution objections, state Superior Court Judge Edward M. Coleman ruled Thursday the jury could determine what weight to give Ernest's experiments.

During a hearing without the jury present, Ernest said testing showed that the shotgun he acquired malfunctioned in 70 of 100 attempts when a wood chip was placed in a specific portion of the firing mechanism.

First Assistant Hunterdon County Prosecutor Steven C. Lember said the test was not relevant, noting Ernest said it could not be replicated outside a laboratory and that Ernest never found a wood chip in that location in Williams gun.

The hearing was watched by Will Griffin, a lawyer for Browning Arms Co., which is based in Mountain Green, Utah. Afterward, he said Ernest often testifies against gun makers and that ``there's no basis in fact for any of the opinions that he's attempting to offer.''

The shooting of driver Costas ``Gus'' Christofi happened as Williams gave a tour of his Alexandria Township mansion to friends and members of the Harlem Globetrotters in the early hours of Feb. 14, 2002.

In addition to aggravated manslaughter, Williams, 36, is charged with altering evidence and persuading his houseguests to lie by saying they were downstairs when Christofi shot himself.

The eight charges carry penalties of up to 55 years in prison. The least of the charges carries a penalty of up to 18 months in prison, but would likely result in probation.

Williams had gone with some friends to see a Harlem Globetrotters game in Bethlehem, Pa. Christofi had driven four Globetrotters from the game to a restaurant near the Williams estate for dinner with Williams and most of the group. They then went to the mansion in Alexandria Township.

Williams retired from the New Jersey Nets in 2000 after a decade in the NBA, unable to overcome a broken leg suffered a year earlier in a collision with a teammate. He was suspended from his job as an NBA analyst for NBC after the shooting.