For the OP:
FYI, the "40" in .40S&W refers to 0.40 inches
, not millimeters.
.40mm would be about the size of a sharpened pencil lead- not optimal for terminal performance.
40mm is a real military caliber, but the guns that fire it qualify as light artillery, not small arms.
Google "Bofors gun". Not very concealable unless you count the inside of a truck trailer.