Military Cartridges
5.7x33mm Johnson Spitfire
The idea for a lite machine gun in .22 caliber did not start with the FN  P90
in 5.7x28mm, which was a hot item in the early 2010's and still today.
It started way back in the 1960's. One of them was the  5.7x33 Johnson
Spitfire, (.22/.30 Carbine) designed by a Boston lawyer and Captain in the
Marine Corps Reserve named Melvin Johnson Jr.. Mr. Johnson turned away
practicing law, to his passion, cartridge/rifle design earlier in 1937 to designed,
built, and successfully tested both a semi-automatic rifle and a prototype light
machine gun (LMG). One of his last postwar ventures was in 1963, when he
introduced a 5.7 mm caliber version of the M1 Carbine, aka "the Spitfire".
Johnson promoted the smaller caliber and the modified 22/30 carbine as a
survival rifle for jungle or other remote areas.
While the concept had some military application when used for in the
selective-fire  M2 Carbine, it was not pursued and very few 5.7mm
Spitfire carbines were produced.  This is basically a military Wildcat, or now a
proprietary cartridge as a few  have been produced commercially.