With both films, which scored low marks from critics, the studio made reviewers agree to hold their critiques until the night before opening.
"This was a very tough weekend," said Paul Dergarabedian, ComScore's senior media analyst. Though there was a "very diverse" selection available at the box office, "a lot of movies have not been well reviewed".
The overall box office total for the weekend was about half that of a year earlier, when Warner Bros' Suicide Squad opened with a blockbuster US$133.7m, ComScore said.
The northern summer's take was about 11 per cent below last year's, Dergarabedian said.
"There just weren't enough good titles to outweigh the negatively reviewed movies," he said.
The Dark Tower, featuring Matthew McConaughey and Idris Elba, was panned by critics, putting a damper on the likelihood of imminent spinoffs, though a TV series is planned. The film generated only 18 per cent positive reviews, said Rottentomatoes.com.
"With review aggregators you can't hide a bad movie any more," Dergarabedian said. "Audiences can turn away from a movie on its first evening."
After the embargo on reviews lifted, analysts at BoxOfficePro.com reduced their estimates on The Dark Tower to US$17.2m.
Elba features as gunslinger Roland Deschain, who has been locked in an eternal battle with Walter O'Dim, played by McConaughey. Deschain's mission is to stop O'Dim from toppling The Dark Tower, which holds the universe together. The film is a sequel to the book series.
Sony and Media Rights Capital were co-producers on the picture, which cost an estimated US$60m.
Meanwhile, acclaim for Dunkirk has kept crowds turning out for the World War II movie from Time Warner Inc's film division. It was forecast to generate US$17.3m.
Kidnap, from California-based distributor Aviron, took in US$10.2m on debut.
The R-rated action thriller scored 41 per cent positive reviews via Rottentomatoes.com. It was forecast to generate US$8m, said researcher Box Office Mojo.
