This issue is a little bit like the 9mm vs. 45acp or glock vs. 1911. Is there a spring set that takes place when a mag stays loaded over a long period of time? Some say yes, and others, no. I've heard awesome arguments for both sides.
Generally, though, it is the repeated loading and unloading that weakens the springs. Thus, keeping a mag loaded for long periods or unloaded for long periods or both okay. Repeated use, however, is what causes wear.
If I wanted to save the future functionality of some mags, I would leave them unloaded until needed. YMMV...
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
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