we still need to know the spine- when you say "500 grains", is that a measured weight, i.e. did you weigh them on a scale? Or does it say "500" on the shaft? tell us everything it says on the shaft.
shafts are usually marked with two numbers:
GPI tells you how much the shaft weighs per inch, this is the shaft alone. it has nothing to do with stiffness.
The other number is stiffness- usually says "500" or .500 or something like that