CLEAR evidence that compression ALONE (without cycling) can weaken a high quality spring.
http://www.funsupply.com/airguns/cocktest.html
Note that ALL of the springs weakened. Note that the weakening was EASILY measured. Note that the weakening progressed the longer the springs were left compressed. I have seen results of similar tests which show identical results, therefore this is a reproducible test.
Furthermore, these are very high quality springs made of excellent materials which have been specially treated to minimize wear and breakage while maximizing spring life. These springs are basically the heart of airguns worth several hundred dollars--they are just about as good as they can be.
It is worth noting that the springs in these airguns are typically coil-bound when the airgun is cocked and the depth of compression can make a BIG difference in spring life. That is also an issue in double column magazines which often compress the spring to the coilbound state.
Basically EVERY authority on spring piston airguns will tell you that minimizing the amount of time the gun is left cocked (minimizing the amount of time the spring is left compressed) will maximize spring life. In fact, one of the prime selling points of the PCP airguns and gas spring airguns is that they can be left cocked indefinitely without fear of damage. Furthermore, it is generally agreed that SHOOTING spring piston airguns (cycling the spring) is not nearly as harmful as leaving them cocked.
AND, if you ask people for their personal experience, you won't have to look far to find someone who can verify that they have seen mag springs weaken from being left loaded--NOT just from cycling.
Here's an experiment: Post that you just bought some new double-column magazines and they are too hard to load. Within a few replies someone will tell you to load them fully and LEAVE them loaded to encourage the springs to weaken sufficiently to allow easy loading. You won't get anyone telling you to cycle the springs to weaken them.
Automobile valve springs are a poor analogy for magazine springs. They are extremely stiff springs--they have to be to stand up to the abuse they go through. Magazine springs could be made that strong and then they would never weaken significantly--but no one could load the magazines!
The factors that determine if leaving a spring compressed is going to weaken it are as follows:
Spring quality. (Self explanatory)
Depth of compression. (Compressing to fully coilbound often reduces life)
Spring design. (Sharp bends in the spring, insufficient thickness, etc.)
Any one of these factors can cause issues, combinations of factors make the problem even more pronounced.
Take a look at a typical single column mag spring and then look at a double column spring. The single column spring typically isn't compressed to the coilbound state with the mag loaded, and usually has smooth radiused bends. The double column mag spring is often completely coilbound when the mag is fully loaded. Also, double column mag springs often have sharper bends at the corners to fit the contours of the magazines.
