My limited understanding is that the 9mm subgun that became the MP5 was originally designed to use the delayed roller locked mechanism adapted from the G3 rifle. The greater the mass moving around in the gun (the bolt group), the more the gun tends to move during operation. The roller locked mechanism slows the movement of the bolt group during the high pressure stage of the burning of the propellent in the cartridge. With a blow back subgun, the shear mass of the bolt keeps the bolt closed during the high pressure phase. Because of the roller lock delay mechanism, the mass of the MP5 is considerably less than a pistol caliber blow back bolt group of the same caliber. So the MP5 has less mass moving around in the gun during operation.
Both the MP5 and the Colt 9mm subgun use closed bolt operating system. So when the trigger is pulled, the hammer is released. When an open bolt subgun trigger is pulled, the massive bolt comes forward to strip a round from the mag, chamber it and fires the round in one motion. The open bolt operation releases the mass of the bolt to move forward in the gun while the operator of the gun is aiming at the target. Again, the greater the mass moving within the gun, the more that mass tends to move the gun. In close quarters that movement tends to move the gun, but since the distance from firearm to target tends to be close, the change from aim to bullet impact is negligible. But the further away the target is, the great the effect of that movement between aim and bullet impact.
During WW II, the Germans developed the first assault rifle (medium power rifle caliber, magazine fed, with safe, semi and full auto capabilities) the MP 43. This rifle was gas operated but was open bolt operated. This made distance shots much more difficult, as the mass the the bolt group coming forward tends to move the gun. Because aimed fire at distance in an important factor in a rifle caliber carbine, the Germans very quickly changed the MP 43 to closed bolt operation (MP 44) which greatly increased accuracy at distance. While a medium rifle caliber carbine has greater range than a pistol caliber carbine, the principle is the same. A firearm with closed bolt operation with a mechanism that would use a mechanical means (delayed roller, piston, or direct impingement) to hold the bolt closed during the high pressure stage, would tend to have lower mass in the bolt group. Lower mass in the bolt group tends to produce less movement in the firearm than with a firearm with open bolt operation using a blow back mechanism. So a carbine that uses a mechanical means to hold the bolt closed during the high pressure stage and uses closed bolt operation, would tend to be most accurate. Of course that would also tend to be the most expensive to produce. The new SIG subgun uses a piston mechanism with closed bolt operation. YMMV.
Scott