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Culture Minister Dario Franceschini led the battle cry after stories about the ad - describing the weapon as a "work of art" - appeared in Italian newspapers.
He tweeted his disgust, saying the image "offends and infringes the law. We will act against the American company, which must immediately withdraw the campaign".
Armalite, a small Illinois-based firm, used a photograph of David, which Michelangelo sculpted from a single block of marble in 1504, in an advertisement for its AR50A1 rifle.
The ad substitutes the muscular Biblical character's sling and stone with the modern weapon. The ad campaign has run in specialised magazines, featuring a photo of the David statue, which is on exhibition in Florence's Academia museum.
Cristina Acidini, superintendent of Florence's museums, was as appalled as the culture minister. "We warn the company not to continue using the image," she told reporters.
Her office told Reuters that Florence museums do not allow use of their art works for commercial purposes without permission and that Armalite had not asked for this.
Armalite did not immediately return a telephone call asking for a comment.
Not everybody in Italy's art world thought Armalite's ad campaign was such a bad idea, however.
"Art is full of violence ... Adulteration of images is not new to contemporary art," said Vittorio Sgarbi, an art critic and former junior culture minister.
After all, some commentators pointed out, 19th-century French revolutionary conceptual artist Marcel Duchamp put a moustache and beard on Leonardo da Vinci's Mona Lisa.