(covers information from several alternate timelines)
The Heckler & Koch MP5 was a submachine gun used on Earth in the 21st century.
In 2024, a SWAT team sent to pacify the residents of San Francisco's Sanctuary District A were armed with either the MP5K, a shortened variant of the MP5, or the SP89, a pistol variant of the MP5K. These weapons were outfitted with mounted laser aiming modules and PDW-style folding stocks. (DS9: "Past Tense, Part II")
In the mirror universe, in 2063, one of the residents living in the community near an old missile silo was armed with the Heckler & Koch SP89, a civilian variant of the MP5K. After Zefram Cochrane murdered the Vulcan captain of the T'Plana-Hath, this resident, with his weapon drawn, called for the seizure of the survey vessel. (ENT: "In a Mirror, Darkly")