The critically acclaimed Sony movie launched to an estimated $40.4 million in North America over the July 26-28 weekend, edging out Inglourious Basterds ($38.1 million) to score the best weekend opening of Tarantino's career (when adjusted for inflation, Inglourious still ranks No. 1 with $45 million). But it will need strong legs to gallop past a B CinemaScore and solid, but not spectacular, exit scores on PostTrak.

CinemaScores are nuanced. While a B grade might be OK in real life, it causes anxiety in the executive offices of Hollywood studios. No other Tarantino film has received less than an A- or a B+ except for Jackie Brown. His two top-grossing titles globally — Django Unchained ($450 million) and Inglourious Basterds ($316.9 million) — earned an A-. His last film, The Hateful Eight, earned a B+ and faltered worldwide ($148 million).

For Sony and Tarantino, the moment of reckoning will come during the movie's second weekend and beyond, as well as the performance overseas, where Once Upon a Time doesn't roll out until August. Tarantino is a particularly huge draw in Europe.

Once Upon a Time in Hollywood is a passion project for Sony studio chief Tom Rothman, and he strategically picked a corridor devoid of big event pics, save for Fast & Furious spinoff Hobbs and Shaw, which debuts Aug. 2. The hope is that the film will play into September, and then get a renewed boost from awards attention.

Set in the winter and summer of 1969, the R-rated film is a twinned tale of a changing Hollywood and the Charlie Manson Family. Margot Robbie plays Sharon Tate, the actress and wife of Roman Polanski who, in real life, was murdered by Manson's followers.

"My feeling is that the B CinemaScore reflects the fact that casual audiences aren't exactly enamored by the inner details of obscure Hollywood lore," says box office analyst Jeff Bock of Exhibitor Relations. "Cinephiles are eating it up, as are most critics, but the average theatergoer craves not these things. And I also think that the Manson elements were played up in a big way without really paying off. That probably turned off casual fans."

Heading into the weekend, some Hollywood studio execs believed Once Upon a Time would ride to $50 million in its debut, considering the combined star power of DiCaprio and Pitt, who have never appeared together before on the big screen. Pitt led the ensemble cast of Inglourious Basterds, while DiCaprio starred in Django Unchained.

Among box office analysts, opinion is divided.

"Naturally, a movie as long and as original as Once Upon a Time In Hollywood will polarize audiences for whom the movie may present more of a challenge than your typical, formulaic 90-minute summer entry," says Paul Dergarabedian of Comscore, who also notes its R rating. "That said, the latest from visionary auteur Quentin Tarantino looks to have perhaps the longest playability of all of his films given the massively positive response from critics, guaranteed awards season attention and is perhaps the most accessible of all of his films."

Once Upon a Time runs two hours and 41 minutes, shorter than Django but longer than Inglourious.

The movie cost $90 million to produce after tax incentives and rebates, making it the maverick filmmaker's most expensive film to date (that doesn't include marketing). By some estimates, it will have to gross $400 million worldwide to turn a profit after production, marketing and talent deals.

Once Upon a Time skewed male over its opening weekend — as all Tarantino's films do — fluctuating between 54 and 58 percent, according to PostTrak. At the same time, it played somewhat younger than both The Hateful Eight and Django Unchained, with 60 percent of ticket buyers between ages 18 and 34. That compares to 57 percent for Hateful Eight and 51 percent for Django. (PostTrak wasn't around when Inglourious Basterds was released in 2009.)

Not surprisingly, Tarantino's latest endeavor performed best on both coasts, with six of the top 10 locations in Los Angeles. It likewise over-indexed in New York City, Chicago, Seattle, San Francisco, Phoenix and Toronto, among other locales.

Inglourious Basterds topped out at $120.5 million domestically, a multiple of 3.2. Django had a multiple of 5.4, although comparisons are tough, since that film opened over the Christmas holiday.

"Some of the greatest movies of all time were not a big hit with audiences upon their initial release and went on to become fabled masterpieces that stood the test of time," says Dergarabedian. "2001: A Space Odyssey, anyone?"