75 years is a long time. Popular mechanics:
In a surprise move, the U.S. Army is asking industry for ideas for a new submachine gun. The last time the Army adopted a submachine gun was in 1943. Itâ€™s not clear why the Army wants a new subgun but it likely has to do with the service’s eventual adoption of a new rifle caliber and new assault rifle.
Submachine guns were developed during the World War I as an alternative to bulky, slow-firing bolt action rifles. Short and firing pistol caliber ammunition, they were ideal weapons for assault troops clearing narrow trenches of enemy troops. The U.S. Army went into World War II with the M1928A1 Thompson submachine gun, which fired the same .45 ACP round as the M1911A1 pistol. Towards the end of the war the Thompson was supplemented by the M3 â€œGrease Gunâ€, also in .45 ACP.
M3 “Grease Gun”
Submachine guns were eventually replaced in many armies by shortened assault rifles, which used heavier assault rifle rounds while still physically compact. In the U.S. Army, the M3 was used up through the 1991 the Gulf War by vehicle and by Delta Force.
According to The Firearm Blog, the U.S. Army has posted a Request for Information from the defense industry for a new submachine gun. The RFI is for a Sub Compact Weapon (SCW) that will fire 9×19-millimeter (9mm Luger) ammunition, have full automatic capability, a Picatinny rail for attaching lights, optics, and other accessories, and mentions the capability to mount a suppressor.