the Army basically dropped the "Grease Gun" in the 1950s in the 1950s, deciding it wasn't that effective.
This needs a little expansion to be correctly understood.
The Army didn't drop the Grease Gun in the 50s, and it wasn't considered "not that effective".
What the Army did (in the 50s) was decide that the idea of a pistol caliber submachine gun for infantry troops was no longer that effective/efficient. Then didn't "drop" the Grease Gun, what they dropped was issuing SMGs to the INFANTRY. The Grease Gun, (both M3 and M3A1) were in active duty service in the 70s, (I know this from personal experience) and I've heard some were still in service in the "sandbox" Gulf Wars in the 90s.
But, they weren't in the hands of the infantry, they were in ARMOR units. Tankers got Grease Guns and .45 pistols. While the Grease Guns were individual weapons and issued from the arms rooms as such, they were considered part of the vehicle "on board" equipment. SO, while not in infantry use, they were not dropped or relegated to special use by small groups, they were is "frontline" use in Armored divisions through the end of the 70s, and in some cases, beyond. This is a fact, I was there (till 10/78), and worked on some of them. An armor company arms room held the machine guns for the tanks and other tracks, 1911A1 pistols for officers and some others, M3/M3A1 grease guns for the tank crews and M16A1s for the rest of the company troops.
During the 50s, a segment of the Army became enamored (some say obsessed) with having one "do it all" rifle. They tried to do it with the M14 and nearly ruined a fine rifle in the process. They wanted something to replace the BAR, the SMG, and the regular infantry rifle (m1 Garand) and the M1 carbine.
The M14 was a good
rifle. It would have been an excellent and superior replacement for the M1 Garand. But it wasn't allowed to be just that. Too big, too heavy and too powerful to be a good carbine and not even close to what makes a good SMG, and at the same time NOT heavy enough to be a good LMG/automatic rifle. Much is made of the short service life of the M14 as if that were in some way due to its failings as a rifle. It wasn't. The MacNamara Defense Dept decided that ALL the military was going to use the m16, whether they wanted to, or not.
The story is out there, well known to some, and widely misunderstood by a lot more.
back to SMGs, "machine pistols", meaning the smallest SMGs are widely favored where open wear of a larger SMG is not allowed. Bodyguards and others who must be armed but cannot appear armed to the public favor the smallest SMG and accept their drawbacks. Larger guns are easier to handle and use, but are too big for anything but open carry.
One hand use of machine pistols is possible, (and dramatic, they love it in the movies) but to me it is the literal definition of "spray and pray!"