Let's suppose you have a 20mm sniper rifle and shoot the internal fuel tank of an F-16 coming directly towards you. What would happen ?
Depends on where it hit the airplane and what other systems were subsequently damaged. A typical rifle bullet will cut through the aluminum structure of an aircraft like a knife through butter. If it was a clean shot where the bullet penetrated the tank and exited the aircraft, hitting nothing else, it would result in minor damage and a small fuel leak, quickly sealed by the tank liner. If the round hit major structural members on its path, this could structurally compromise the aircraft. Most dangerous would be a hit where the bullet penetrated the tank, then exited and pierced the hot section of the engine. This would allow hot exhaust gases to escape the engine and could easily cut into the fuel tanks like a blowtorch, causing internal fires and possibly the loss of the airframe.