It's not the compression of a spring that causes a spring to loose strength. It's the spring going through it's compression/relaxation cycles that will cause a spring to loose strength. Example: There was a WWII veteran in Memphis about ten years ago that shot two perps that forced their way into his home. The pistol he used had been loaded with 230 gr ball ammo since 1945. One BG DOA, the other was well perforated (Multiple hits on target). Load it up and don't worry about it. If and when the spring starts to go weak just replace the spring with a good quality spring. Personally I replace the springs in my magazines with a new Wolff spring when the slide catch starts not engaging after the last round out of the magazine and not before. When buying a used pistol I always replace the recoil spring as a matter of habit. In 49 years I have replaced way many more recoil springs than I have magazine springs.
I agree with the above comments. With good quality springs, which S&W are, you can leave the mags loaded indefinitely without a problem. I left a GI 1911 mag loaded since 1974 and found it in a box last year. It shot fine.
Even if a spring does weaken, get some Wolff springs and you are good to go for a long time.
If you change your S&W magazine springs after 5 years or 5,000 rounds, you will never have a problem, as they last longer than that. Loaded magazines are considered in use.
does it hurt to leave a mag full for a long time
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