Depp, 46, told the Chicago Tribune the World War II-vintage submachine gun belonged to a relative.
"I butted it up against the tree because it tends to ride up on you," Depp told the newspaper, adding that his father had helped him keep the weapon level from the time he was only 5 or 6 years old.
The Tribune reported Saturday that while Depp has handled firearms since he was a lad, he is one of the rare actors who reached Hollywood's top tier without a string of action shoot-'em-ups on his resume.
That will change with next week's opening of "Public Enemies," in which he portrays Dillinger, the heavily armed Depression-era bank bandit who made headlines with his daring heists.
Rosie O'Donnell unveils nearly 50-pound weight loss
Justin Bieber crashes Drake Bell's album release party