The FN 5.7×28mm (designated as the 5.7×28 by the C.I.P. and FN 5.7x28mm NATO) is a small-caliber, high-velocity, smokeless powder, rebated rim, bottlenecked centerfire cartridge designed for handgun and personal defense weapon (PDW) uses manufactured by FN Herstal. It is similar in length to the .22 WMR (5.7×27mm) and to some degree similar also to the .22 Hornet or .22 K-Hornet. Unlike many new cartridges, it has no parent case; the complete package was developed from scratch by FN.
The 5.7×28mm was developed in conjunction with the FN P90 PDW and FN Five-seven pistol in response to NATO requests for a replacement for the 9×19mm Parabellum cartridge. In 2002 and 2003, NATO conducted a series of tests with the intention of standardizing a PDW cartridge as a replacement for the 9×19mm Parabellum cartridge. The tests compared the relative merits of the 5.7×28mm cartridge and the 4.6×30mm cartridge, which was created by Heckler & Koch as a competitor to the 5.7×28mm. The NATO group subsequently recommended the 5.7×28mm cartridge, citing superior performance in testing, but the German delegation objected and the standardization process was indefinitely halted.
By 2006, FN's 5.7×28mm firearms—the P90 PDW and Five-Seven pistol—were in service with military and police forces in over 40 nations throughout the world. In the United States, 5.7×28mm firearms are currently used by numerous law enforcement agencies, including the U.S. Secret Service.
In addition to being used in the FN P90 and FN Five-seven firearms, the 5.7×28mm cartridge has subsequently been used in a number of other weapons, such as the AR-57 and FN PS90 carbines. Excel Arms has developed four firearms chambered in 5.7×28mm, MasterPiece Arms offers three different firearms in 5.7×28mm., and CMMG offers several of its AR-Style Banshee firearms in 5.7x28. As of December, 2019, Ruger offers its Ruger-57 semi-automatic pistol chambered in this cartridge. January 2021 saw the announcement by Kel-Tec of the P50 handgun, which uses 50 round P90 magazines. Palmetto State Armory introduced its Rock 5.7 pistol in January 2022; it became available for purchase in May 2022.
The 5.7×28mm cartridge itself is produced in a number of varieties, two of which—the SS195LF and SS197SR—are currently offered by FN to civilian shooters.