The bullets that struck President Reagan and two of the three other persons wounded in Monday's assassination attempt were positively identified yesterday by the FBI as "Devastators" -- expensive, customized .22-caliber cartridges designed to explode upon impact with the force of slugs fired from much more powerful handguns. None of those bullets exploded, however.

A "strong probability" exists that a fourth bullet, which struck White House press secretary James S. Brady in the head, was a Devastator that did explode.

The FBI said last night after further tests that the explosive container in the bullet that hit Reagan was still complete and that none of the material had leaked into the president's chest as was first feared.

Although federal authorities said they were not familiar with Devastators, a check of 21 ammunition dealers in the Lubbock, Tex., area -- where alleged assailant John W. Hinckley Jr., purchased three handguns -- shows that exploding ammunition, including Devastators, is readily available.

Lubbock pawn shop owner Scott Elmore said exploding bullets were the "in thing" a year ago for hunting rabbits and were sold at most gun stores. They are usually used in rifles. The bullets, however, were not reliable, Elmore said, and their popularity diminished. At least one major Lubbock ammunition dealer, K-B Guns, still sells Devastators, primarily to police.

Thomas Kelleher, assistant FBI director in charge of the agnecy's laboratory, said the bullets removed from Secret Service Agent Timothy J. McCarthy and D.C. policeman Thomas K. Delahanty also were identified as Devastators.

The crime lab chief said it is unclear why the bullet that hit Brady exploded while the three other slugs did not. An official for Bingham Ltd., the Norcross, Ga., company that manufacturers Devastators, told the FBI that the bullets had to be traveling at least 900 feet per second to burst on impact.

One possible reason the slugs didn't explode, a ballistics expert suggested, is because the RG14 six-shot revolver which Hinckley is accused of using had a 1 3/4-inch barrel, far too short to allow the bullet to reach its full power.

Sandy L. Brygider, director of Bingham Ltd., said Devastators were created for "defensive use." They were designed primarily for use against skyjackers by skymarshals who needed a round that could be fired aboard an airplane without the risk of penetrating the aircraft skin and causing rapid decompression. The bullets have been available commercially since 1978.

Technical experts at Bingham Ltd., said they enlarge the hole in the tip of a standard .22-caliber long-rifle hollow-point bullet and insert a small aluminum cup which is filled with from one to three grains of lead azide. The tip is sealed with lacquer. The bullet is designed to explode on impact when the lead azide is crushed.

Asked if the bullet is more or less deadly than a convention .22 bullet, Kelleher said, "I really don't know but, normally, fragmentation causes more damage. It's made to reduce penetration and keep all the punch that you can."

A company spokesman for Devastator said the bullet has the impact of one shot from a .38-caliber or larger handgun.

Kelleher said it was possible but not probable that the bullet in President Reagan's lung could have detonated or exploded during the operation to remove it.

Noting that the bullet had hit the president's car before striking him, Kelleher said, "It already had taken as big a shock as it was going to take on the ricochet."

Delahanty underwent emergency surgery Thursday night at the Washington Hospital Center to remove the bullet lodged near his spinal cord after the FBI had told his doctors it might be explosive. "Our concern was the bullet's location in the neck near the spinal column," Kelleher said. He added that there were concerns that the bullet still might ignite because of heat or shock from some hospital treatment. $ Kelleher and FBI spokesman Roger S. Young defended the length of time it took for the FBI to determine that the bullets were Devastators. The Washington Post reported yesterday that an empty box labeled "Devastator" was found in Hinckley's hotel room by investigators Monday. But Kelleher said the bureau had not heard the name "Devastators" until Tuesday.

There was further delay because not a single reference about Devastators was found in the 3,000 cases on file in the bureau's firearms lab, he said. After a full day's search Tuesday, it was learned that the manufacturer was the Bingham company, and a request was sent to the manufacturer for specimens of the shells, Kelleher said.

The cartridges didn't arrive until Thursday afternoon and after a quick examination Delahanty's doctors were informed about the possibility that he was still carrying a potentially explosive round.

Kelleher noted that it was also likely that the two bullets that did not hit anyone were Devastators as well. One hit the president's limousine window and fragmented. The other hit the window of a building across the street and fragmented.

A source said, however, that FBI agents were told two hours after the shooting that the bullets fired at Reagan could not have been conventional ammunition because the bullet hole in the glass window across the street was too large for a .22-caliber and because pieces of the bullet were found on the floor between the glass and a drape.

Although the FBI did not have any records about Devastators in its files, the D.C. Police Firearms Unit has a box of Exploders made by Bingham in its bullet library. Exploders are made exactly like Devastators, except they are a larger caliber bullet than .22s.

The Devastators fall under the same regulations as less powerful conventional bullets even though they have exploding heads. A bullet must have at least one-fourth of an ounce of explosive in its tip to come under stricter federal rules governing artillery shells.