The sight of a Stallone or Schwarzenegger machine-gunning a roomful of bad guys may become a thing of the past -- but not because film critics are getting a say in movie production.

Instead, the federal government is responsible: Last month, President Reagan signed Public Law 99-308, which prohibits the sale or manufacture of machine guns except for use by police or the military. Because of a last-minute amendment to the bill by Rep. Larry Smith (D-Fla.), it also outlaws machine guns that are altered to fire only blanks -- i.e., the kind widely used in movies such as "Rambo" and "Cobra" as well as on a certain Friday-night TV show that brings lots of attention to Smith's home state. Unless it is changed -- and there already has been talk about doing just that -- the law will force producers wanting to use high-tech automatic weaponry to shoot their films outside the United States. Politics on Stage

"Circe & Bravo" is a controversial play about a first lady of the United States who has what its author terms "a sexually and politically explicit encounter" with one of her Secret Service agents. It's playing in London in a production directed by Harold Pinter and starring Faye Dunaway -- who, according to playwright Donald Freed, has just purchased the film rights to the production.

Much of Freed's previous work also has had a political bent: He cowrote "Executive Action," a film about the Kennedy assassination, and wrote "Secret Honor," a play about Richard Nixon. Before it hits the big screen, "Circe & Bravo" will play London's West End and Broadway (in 1987), followed perhaps by Los Angeles and Washington engagements. Enough Is Enough

Members of the Ontario Film & Video Review Board -- which views motion pictures that will be shown in the Canadian province to determine their suitability -- acted more like disgruntled theatergoers than reviewers when they came up against the action film "Toxic Avenger" recently -- they walked out on the film. The five members of the board ordered the film stopped after seeing its first 40 minutes, less than half of its running time; then, they banned it in Ontario. The official report spelled out the reason: " . . . because of graphic and prolonged scenes of gratuitous violence, torture, crime, cruelty, horror, rape, degradation to humans and animals and exploitation of the blind and children, as well as indignities to the human body in an explicit manner." Countered Lloyd Kaufman, president of Troma, the film's distributor: "We watch what Sylvester Stallone and Eastwood and Schwarzenegger do and we take our cue from them." %(SECTION)ummer's Slump

Now that we're all of four weeks into the official summer movie season, enough returns are in for Daily Variety to make its first projection of overall summer business. The news isn't good: Based on attendance so far, the publication forecasts a $1.35 billion summer box-office gross, down about 6 percent from last year's lackluster figures and scanty enough to make it the worst summer in five years.

If Variety's projections are wrong, it'll be because of films like "Back to School," which was the clear box-office champ last week. At first, it looked as if Paramount Pictures was making a canny move by opening "Ferris Bueller's Day Off" on Wednesday, giving it a two-day head start on "Back to School" and leading to substantial returns those first two days.

But come Friday, Rodney Dangerfield showed he can out-muscle John Hughes: His comedy about returning to academia made $8.9 million compared with $6.3 million for "Bueller," and in per-screen averages it also handily beat "Bueller" and Paramount's third-place film, "Top Gun." Those three movies, meanwhile, far outdistanced everything else in release, which means that "Cobra," "Raw Deal," "Poltergeist II" and "SpaceCamp" are quickly running out of gas, and the market needs a quick shot from some new films to keep the dire forecasts from coming true. "The Karate Kid, Part II" and the Robert Redford-Debra Winger comedy "Legal Eagles" ought to help when they open tomorrow.