Solid core inductance

[[527]phil]

Hello everyone, first off I want to say that I've searched all over the place, and I even asked my mentor/physics teacher about this. For my senior project I'm doing high voltage electro magnetics, and I'm going to build a coil gun. I've built them before but they weren't very efficient. I've been trying to find a formula to calculate the inductance of a coil wrapped around a solid core. I've only been able to find this one.
But the only problem I'm really having is determining the permeability of the medium. I'm using the core of a gutted transformer from an ATX power supply. I checked the markings on the outside but came up empty. I can't just calculate the inductance as if it was an air core coil because the final data would be incorrect. Can anyone tell me how to find the permeability of any given material, or if I could but a core with a known permeability. If not then I guess I have to make an air core coil.