One expendable character is incinerated by a letter bomb and three others are mowed down in a gun battle before the credits of the aptly named "Firepower," now at area theaters. Director Michael Winner sustains this downbeat of mayhem with remarkable consistency, so that it's a rare stretch in "Firepower" when someone isn't getting roasted or ventilated.

Audiences in the Mood for action-packed junk are not cheated by "Firepower." If it's gratuitous, it's also energetic, an animated example of yahoo wish fulfillment. The formula is "Mission: Impossible," and the disreputable hero figure embodied by James Coburn also derives from an action staple of the '60s - the Derek Fint character played by Coburn in three movies. Derek Flint, in turn, was inspired by the success of the James Bond series.

The Flint type now has evolved into a former bounty hunter and Mafia hit man named Jerry Fanon, who is induced to come out of retirement (where Sherlock Holmes turned to beekeeping, Fanon has turned to botany) to aid an old mobster friend, Eli Wallach, compelled to act as a go-between by the Justice Department.

The Feds want to arrange the abduction of a nefarious financier called Stegner who has taken refuge from several warrnats at a well-guarded private estate on a Caribbean island. If Wallace arranges the deal, his own felonies will be forgiven, and since Coburn owes Wallach an old debt . . .

Coburn engages O.J. Simpson, a cheerful safecracker, as his invaluable sidekick for Mission Stegner. Coburn also plays Fanon's double, a decoy named Eddie who comes in handy whenever Fanon needs to be in two places at the same time. Sophia Loren co-stars as the ambiguous heroine: the widow of the letter bomb victim, her true allegiance remains in doubt right up to the fade-out.

Although Coburn and Simpson are shown disabling enemies with tranquilizing darts, they are differentiated from the bad guys mainly by superior technical proficiency. It's not that they're more decent, God forbid: they're simply more expert at commando operations.

The caribbean locales and Gato Barbieri score contribute rather more attractive forms of distraction, and amusing familiar faces keep popping up: Jake La Motta as Coburn's butler, Billy Barty as a Caribbean casino owner and finally Victor Mature, beaming but almost unmanned by a ludicrous coiffure, in a last-minute throwaway walk-on. As worthless movies go, "Firepower" recommends itself by rarely letting up. CAPTION: Picture, Sophia Loren and James Coburn in "Firepower"