The Sun newspaper in London said Middleton was with three male companions in an Audi convertible when the driver took out the alleged weapon and pointed it at a French photographer following them.
The Paris Judicial Police are investigating the matter and it is possible Middleton could face charges if the gun turns out to be real, a source told The Sun.
"If the evidence points to her involvement, she will be prosecuted," the insider said. "Anybody involved in the illegal use of a handgun in public is liable to arrest and interrogation."
Prince William's mother, Diana, was killed in a 1997 car crash when her driver, who was intoxicated at the time, attempted to allude paparazzi following her in Paris.