Convicted heroin smuggler Robert L. Stuckey, who has been cooperating with government investigators since he pleaded guilty 11 months ago to conspiring to murder a federal prosecutor here, was sentenced yesterday to serve seven years in prison for the conspiracy.

Senior U.S. District Court Judge Howard F. Corcoran said he considered the sentence to be "light under the circumstances," but had changed his mind about a stiffer penalty after representations about Stuckey were made to him during a private bench conference with government officials at yesterday's hearing.

Corcoran ordered that the seven-year sentence follow a minimum 10-year term that Stuckey, 43, is serving on the heroin-smuggling conviction.

Corcoran sealed the court record of the bench conference. Assistant U.S. Attorney Robert R. Chapman said in open court that the discussions concerned a plea bargaining agreement with Stuckey in May 1980. Chapman said since then Stuckey has been "helpful" to the government in its ongoing investigation into the shooting of former assistant U.S. attorney Barry L. Leibowitz. The prosecutor was slightly wounded as he walked through a parking lot at the federal courthouse here on Dec. 20, 1978.

At the time, Leibowitz was investigating a major international drug smuggling ring, involving Stuckey and others, who prosecutors say imported more than $30 million in high-grade heroin into this country.

During his trial on drug charges in October 1979, Stuckey testified that the alleged kingpin of the drug operation, Linwood Gray, told him he had shot Leibowitz. At a separate trial, Gray was acquitted of criminal charges involving the heroin ring, but was convicted of tax evasion.

When Stuckey pleaded guilty to the charge involving the Leibowitz shooting, he said he conspired with both Gray and a second man, Joseph F. Wilson Sr., to kill the prosecutor. Wilson was also acquitted of narcotics charges in connection with the heroin importation scheme. No charges have been brought against Gray or Wilson in connection with the shooting of the prosecutor.

When the government reached an agreement with Stuckey for his cooperation, it was described by federal officials as a major step towards apprehending the persons directly responsible for the Leibowitz shooting. However, no further charges have been brought in the case in the 11 months since Stuckey entered that agreement and pleaded guilty to the conspiracy charge.

At the time Stuckey entered his guilty plea, the government said it could prove that Stuckey once tried to lure Leibowitz to the D.C. Jail, intending that he be shot while en route, that Gray told Stuckey several times that "Leibowitz should be killed" and that at Gray's direction he retrieved a .38-caliber pistol and silencer that had been used to shoot Leibowitz but abandoned in a car near the courthouse.

Stuckey picked up the gun, which he later turned over to federal authorities, and then set fire to the car in a department store parking lot, prosecutors said.

Prosecutor Chapman told the court yesterday that a local lawyer, whom he did not identify, gave Stuckey and others information about Leibowitz's home address, personal friends, automobile and methods of travel. In addition, Chapman said in court that Stuckey helped acquire or attempted to get a variety of items to be used in connection with the slaying conspiracy, including poison heroin, cyanide, handguns, a laser beam with lethal capabilities and flammable chemicals for destroying evidence.