How bullet proof is 7 inches of steel

Can you penetrate a 7 inch steel bunker re-enforced with steel I beams with an ac130? Or any other bomb other than nuclear bombs

Yes. It doesn't matter what the airplane is (and if you mean something other than the gunship - give details), it does matter what the shells are. There are shaped charge weapons that can be carried on the plane (or on a Hummer) that will put a flaming hot slug of metal through 7 inches of steel. For this reason, modern tanks have ceramic and fiber armor added to the steel to deal with modern weapons.
http://www.google.com/search?q=ac130&ie=…