Attorneys for Prince George's County Deputy Police Chief Joseph D. Vasco Jr., accused of "ambushing" an unwitting robbery suspect at close range in 1967, contended in federal court today that Vasco fired in self-defense from at least 10 feet.

Using ballistics evidence in conjunction with earlier testimony by Maryland's chief medical examiner, the attorneys sought to show that a 7-inch-wide shotgun pellet pattern in the abdomen of the slain suspect was consistent with that of a 12-gauge shotgun fired from 12 feet away.

Examiner Russell Fisher testified earlier that the suspect, identified as William H. Matthews, 18, was hit by Vasco's 12-gauge gun from a distance of 10 to 16 feet. Fisher's testimony was calculated to offset claims by the plaintiffs suing Vasco and two other officers that Vasco gratuitously shot Matthews with "murderous force" in a close-range "coup de grace" after Matthews had already been mortally wounded by another officer.

Matthews' family and the family of another man shot and killed by police in a series of so-called "death squad" holdups of convenience stores in 1967 have sued police for $9 million, claiming the officers directed informants to recruit participants for the holdups and then "lay in ambush" for them at the targeted stores.

Police have denied the allegations, contending the informants came to them with tips about planned holdups and officers then routinely staked out the stores.

Matthews, who was armed, was shot and killed on June 8, 1967, during an attempted robbery of a High's dairy store at 9101 Riggs Rd. in Adelphi. Plaintiffs say he was lured to the scene by informant Gregory Gibson on instructions from Vasco. Vasco has said he shot Matthews as Matthews crawled toward a pistol that had been knocked out of his hand by the impact of the first blast of shotgun fire.

Plaintiffs' attorney R. Kenneth Mundy challenged today's ballistics evidence, contending that horizontal test firings into a perpendicular target are significantly different from the actual shooting in which Vasco fired downward at Matthews on the floor of the store.

In another challenge to the plaintiffs' case against the police, a former 7-Eleven store clerk testified about a bizarre holdup in 1966 that ultimately led to the arrest of informant Gibson.

Defendants say the clerk's testimony undermines Gibson's claim that he was never involved in a robbery before the High's 1967 holdup when he was an informant.

Michael N. Hoffman, 37, said a man answering Gibson's description entered the 7-Eleven on University Boulevard where Hoffman was a clerk, brandished a chrome-plated pistol and cleaned out the cash registers. Just as he finished filling a paper bag with the cash, Hoffman said, an "Oriental" customer came into the store for a cup of coffee, apparently unaware of the holdup. Hoffman said he asked the robber to give back some of the money "in case I had to make change for the customer."

The customer completed his purchase and left. Then, Hoffman said he turned to the robber, who by now had pocketed his pistol, and asked to see it again, telling him he "didn't believe it was real." Hoffman said the robber showed him the weapon and Hoffman peered down its barrel.

"I know it's hard to believe," Hoffman told the jury. "It was stupid . . . . But I was just a kid, 20, you know." He said his curiosity was satisfied when he saw what appeared to be "real bullets" in the pistol. The robber then started to leave but turned at the last moment and asked Hoffman for a carton of cigarettes, Hoffman said. Hoffman complied, and the robber left.

Two months later, Gibson was arrested but charges were dropped after he became a police informant.