Basically, any weapon that can be fired quickly can do it better.
For those that understand a bit of coding:
As soon as the weapon is on the ready state and the WeaponReady action occurs, you can change weapon then fire with the one you got.
The classes all have ACS to forcefully take the weapon off, so, as soon as you shoot with the new weapon and it gets taken away, you'll go back to your old weapon, thus leaving to weirdness and "machinegun" effects. It isn't MM6 specific, any class can do it, but rapid firing classes can do it better as you can change the weapon faster.
Spoilered for space.
One easy fix for this would be not removing weapons you get when you shoot, making the machinegun effect worthless.
I'm sure there are other fixes, but this is all I'll say for now.