The infamous baseball scene in The Untouchables was based on reality

One of the interesting facts about Al Capone is the infamous baseball scene in The Untouchables was based on reality. Capone usually had his minions do his dirty work, but he did take matters into his own hands on occasion. Consider when a Sicilian gangster named Joe Aiello persuaded some of Capone's own—Chicago Outfit gangsters Albert Anselmi, John Scalise, and Joseph Giunta—to overthrow the gangster, and one of Capone's bodyguards, Frank Rio, discovered the plot.

Scarface invited the men to dinner, as author John Kobler writes in Capone: The Life and World of Al Capone—the traditional "hospitality before execution." He revealed to them after the meal that he was aware of their treachery. The men were then tied to their chairs by Capone's bodyguards, and Capone stood up, grabbing a baseball bat:

“Slowly, he walked the length of the table and halted behind the first guest of honor. With both hands, he lifted the bat and slammed it down full force. Slowly, methodically, he struck again and again, breaking bones in the man’s shoulders, arms, and chest. He moved to the next man and when he had reduced him to mangled flesh and bone, to the third. One of the bodyguards then fetched his revolver from the checkroom and shot each man in the back of the head.”

Later, in The Untouchables, Brian De Palma used the incident to show Capone (Robert De Niro) treating some of his colleagues to a lavish dinner before murdering a guest with a baseball bat himself.