Handgun rounds have no trouble penetrating an aluminum aircraft fuselage, unless it bounced off a structural brace. But, all you would get is a lot of noise, as the aircraft pressurization system would have no problem keeping up with the leak (it's air from the compressor section of the engine(s)).

The primary danger is if the round impacts something important while exiting the fuselage. There's a lot of wiring and/or hydraulic lines weaved throughout the fuselage. There is redundancy, but elimination of a redundant component introduces a flight-safety issue.