I know Spain has a strong hunting tradition but in regards to the law I have no idea.
You may want to contact the Spanish Embassy in Washington or even the US Embassy in MAdrid to get the most current and up to date info.

Since the guns are checked baggage and if you are only transfering flights in Spain, I don't think you will have a problem.
I would recommend doing your homework 1st however.
Good Luck and have fun on your trip.