They start out missing a “BAR” and wind up behind bars.

Hard-luck — and mostly drunk — gamblers at the Resorts World Casino in Queens are venting their frustration on its slot machines, bashing and breaking dozens of the one-armed bandits each year, police sources told The Post.

From the October 2011 opening of Gotham’s only legal gambling house through the end of last month, cops have made 491 criminal mischief arrests at the casino — the majority of which were of bitter bettors who battered slot machines, the sources said.

“They’re very fragile, and they’re easy to break,” a police source said of the more than 6,000 machines that fill the hall.

“Sometimes, guys punch it hard, other times, they just tap it. Sometimes they can’t believe [they broke them].”

When the raging wagerers take their whacking too far, the casino calls the cops and gives the culprits a chance to cough up the cost of fixing the machines — from $600 to $1,400 — or be busted for criminal mischief, the police source said.