Your mind is flapping again, ENT.
It's funny that you should mention armor-piercing shells, because the plane that flew into the Pentagon behaved like one of them. Except it was constructed quite differently, so it behaved differently.
Instead of being dense and ultra-hard, it was light and semi-rigid, so the effect was for the plane (as it was travelling at speed) to compress and create a relatively small hole in the armored building. Which is exactly what we saw.
