I have wondered about this same question. On the FXO mag I just had GT work on, the spring was too weak to feed the bullets. Other then that, it was in good shape. What happened?
Here is a interesting article about this issue from a physics point of view
https://www.physicsforums.com/thread...spring.661309/
Based on this article,perhaps the material used on that mag spring was sub par(late war production issues?) or it was exposed to heat?
Bob