Fired at the muzzle, a representative .40 S&W load (11.7 g, FMJ, 300 m/s) reaches roughly 8 mm (0.3 inches) into aluminum 6061. That is enough to defeat a typical barrier of this material — treat anything behind it as exposed.
About Aluminum 6061
Aluminum plate (~6 mm), as found in light vehicle and aircraft skins.