Are 40 caliber bullets supersonic

Some .40 caliber bullets are supersonic and some are not; it depends on the load. Generally lighter bullets (155 grain or less) will be supersonic, and heavier bullets (165 grain or more) will be subsonic, although there are a couple 165 grain bullets out there with advertised velocities about equal to the speed of sound.

No matter what bullet you purchase, if you feel like subsonic/supersonic is a difference-maker to you, then check the manufacturer's specifications on the ammunition you're purchasing.