“The accepted wisdom seems to be that leaving magazines loaded does not hurt the magazine spring. The loss of stiffness comes from cycling the magazine springs. This has always made sense and seemed to agree with most of my engineering classes. Creep is a material property that you see most often in plastics and with modern heat treating should be pretty much none existent in good spring steels. Well this small experiment seems to point to the possibility of other conclusions:”

http://mcb-homis.com/magspring/