The Day of the Boomer Dukes by Frederik Pohl

A bored resident of the future, reading a crime magazine from the 1950s, decides to time travel back to the 20th century with a suitcase full of weapons and offer his services to the Mafia. Just for a little excitement.
Just as medicine is not a science, but rather an art-a device, practised in a scientific manner, in its best manifestations-time-travel stories are not science fiction.
Time-travel, however, has become acceptable to science fiction readers as a traditional device in stories than are otherwise admissible in the genre.
Here, Frederik Pohl employs it to portray the amusingly catastrophic meeting of three societies.
The Day of the Boomer Dukes was first published in Future Science Fiction No. 30 1956.
Total Running Time (TRT): 45 min.
Frederik George Pohl, Jr. (1919-2013) was an American science fiction writer and editor, with a career spanning more than seventy-five years. From about 1959 until 1969, Pohl edited Galaxy and its sister magazine If; the latter won three successive annual Hugo Awards as the year's best professional magazine. He won four Hugo and three Nebula Awards.
The Science Fiction Writers of America named Pohl its 12th recipient of the Damon Knight Memorial Grand Master Award in 1993 and he was inducted by the Science Fiction and Fantasy Hall of Fame in 1998.