Colloquially referred to as the "Hush Puppy", the MK22M0 featured a locking slide which prevented action noise and blowoff gasses, as well as preventing spent casings from ejecting.
Using specially designed 158gr 9x19mm ammunition, with the slide locked in place, the pistols report could be as low as 129db (a jackhammer is typically 120-130db, unsupressed standard-velocity 22lr is 140-150db).