But the op asked about 300 BO subsonic. If you launch a 30 caliber 230 gr bullet or a 45 caliber 230 gr bullet ~1000 fps, you have the same ballistics. The only difference is the 30 cal bullet is going to need to really open up, or it will be zipping on through your walls off into the neighborhood.
While our military has mostly moved away from smg's, the Tommy Gun and Grease Gun seemed to have worked pretty well in WWII.
