The Navy, with a big assist from FBI laboratory work, has concluded that Gunner's Mate 2nd Class Clayton M. Hartwig "probably" placed some kind of igniting device between powder bags inside a 16-inch gun of the battleship USS Iowa, causing a premature explosion that killed Hartwig and 46 fellow sailors, defense officials said yesterday. Hartwig was serving as gun captain April 19 when the No. 2 turret blew up, they said, meaning he was closest to the breech, the best position to booby-trap it without being observed. A device that would be triggered when the gun's rammer pushed the silk bags of powder together, or a timing device, could easily set off the powder, experts said. If Hartwig actually did this, as the Navy suspects on the basis of its lengthy investigation and exhaustive FBI examination of the residue in the gun barrel and surrounding area after the horrific fireball tore the turret's interior apart, the gunner's mate would have known he would be the first to die as flames shot out of the open breech and engulfed him. This finding of probable cause is scheduled to be disclosed today when the Navy releases a 1,100-page report. The long-awaited report also recommends disciplinary action because officers failed to make sure the three guns in the No. 2 turret were maintained properly. It also says that some crew members were not fully trained for their jobs, officials said. Also, the gunpowder for the Iowa was stored improperly, the report states. Under Navy procedures, Rear Adm. Richard Milligan, who headed the probe, was required to give his opinion on what caused the explosion. His finding that Hartwig sabotaged the gun is in that context, officials said, as distinguished from a hard conclusion. Hartwig, 24, became a focus of naval investigators' attention after they discovered he had named another Iowa sailor, Kendall Truitt, as a beneficiary of a $100,000 life insurance policy. Investigators began pursuing a theory that Hartwig might have been suicidal because of the reported breakup of a relationship with Truitt. Hartwig's family has bitterly denounced this aspect of the probe. Truitt has publicly denied any homosexual relationship with Hartwig or anyone else. One ordnance expert said Hartwig could easily have concealed a small detonating device in the 5-by-5-inch metal foil pack that is placed in the breech during every firing of the 16-inch gun. The pack melts when the powder burns, releasing a gas that sweeps out behind the projectile and cleans the barrel. The actual device or technique used in the April 19 blast is not known, officials said. Navy officials briefed some aides on the outlines of the report. They were told, defense officials said, that Hartwig is suspected of committing an "unlawful and illegal act" but were not given details. The staff members were also told, officials said, that "foreign" material was detected in the material left in the barrel and wreckage in the turret. Navy investigators at first feared the damage to the 16-inch gun and turret was so extensive that the cause of the explosion might never be determined. The material gleaned was subjected to thousands of laboratory tests by Navy and FBI explosive experts, Pentagon officials said. Residue in the barrel provided the key clues after thousands of tests, they added. "The FBI did some amazing work in their laboratories," one official said. In an investigation that went down a separate track, the Naval Investigative Service (NIS) obtained statements from Iowa crew members about Hartwig's behavior, including assertions that he threatened to commit suicide and even boasted of buying a detonating device. These suspicions were hardened by at least one statement by an Iowa sailor that Hartwig had showed him a timing device he had bought that could sink the battleship, sources said. Hartwig's sister, Kathy Kubicina of Cleveland, told The Post early in the probe that NIS investigators and FBI agents questioned her closely about Hartwig's relationship with Gunner's Mate 3rd Class Truitt, one of the 11 sailors in the bottom of the seven-deck turret who survived the fireball and blast. Seaman David Smith charged this week that Navy investigators pressured him into saying Hartwig had made a sexual advance to him and had talked about how to set off an explosion with a cheap timing device. The Iowa is one of four battleships built for World War II duty and brought out of mothballs in the Reagan administration. Each ship carries three turrets of 16-inch guns as well as Tomahawk cruise missiles. When President Bush sent an armada to the waters off Lebanon last month after terrorists hanged Marine Lt. Col. William R. Higgins, the Iowa was among the warships ordered to be ready to deliver a retaliatory blow. Bush decided against such retaliation.