Originally Posted by That_Is_My_El_Camino
Does the same apply to leaving a magazine loaded at full capacity or near full capacity?
It should. There are three modes of failure that are very important to spring design. Material strength (yield), fatigue and creep.
On material strength, you need to know the max stress on the material when the spring is fully compressed. That needs to be less than the yield point of the material (when it goes from elastic deformation to plastic/permanent deformation). Keeping it below the elastic limit will ensure that the material will always return to its original state.
Next is fatigue. Springs in guns (I would guess) are subject to high-cycle fatigue (loads do not exceed the elastic limit). If they are designed to keep the stresses below the fatigue threshold, springs can be designed for "infinite fatigue life"
Lastly is creep. Lots of people will tell you that springs slowly weaken with time when compressed. In reality, spring steel has a very high resistance to creep when not overloaded. Creep generally comes into play when you get into high stresses and temperatures.
In short, if a manufacturer is worth their salt and puts any thought into their designs, it shouldn't be a problem.