In the original 'Day of the Jackal' film (not the horrendously bad Bruce Willis nonsense of a similar name) the rifle used was chambered in .22 Magnum, or possibly .22 Hornet. Whichever was the case, the loading was definitely subsonic, as no supersonic crack was heard when the shooter was sighting in the suppressed rifle at a melon target. The point seemed to me to be allowing a slightly larger volume of mercury in the bullets intended for use in assassinating Charles de Gaulle, the bullet size being slightly longer than would be practical in .22lr. Ultimate quiet and precision were central to the role of that rifle, hence the use of a .22" round instead of something larger in diameter - the suppressor used had to be of a relatively small outside diameter for concealment in transit. I can think of no other practical implementation of a subsonic .22 Magnum cartridge... but perhaps someone will come along and explain why such a thing might be wanted.