Hi Tony

Here is a reference to a stock: http://www.cnczone.com/forums/rhino_3d/32789-rifle_stock.html
Note all the red cross sections.
You could measure your stock prototype, with respect to some reference lines/planes, like the butt end, "centerline", angles, etc ...
And then create a bunch of cross sections, to be entered into MoI, with the correct dimensions...