WD-40 is for sissies; actually it is for drying ignition wires, originally, Water Displacer recipe #40.
Get some REAL penetrating oil, Kano Kroil is the best, PB Blaster is second best. Spray it on, tap with a hammer (the vibrations help it penetrate), spray more and WAIT for it to work. Heating the shell with a hair dryer may also help during removal.