This is nothing new. During WW2 there weren't enough m1927 Thompson sub machine guns to go around. It was perfect for jungle warfare - a compact full auto weapon firing a pistol round. Great for CQB.
GM came up with a simplified .45 acp submachine gun that was lighter, cheaper to manufacture due to stampings and less machining and it had a lower rate of fire. The Army bought this "Grease Gun" off of them in large numbers.
