There was study by a gun magazine a few years ago in which they performed the following:
(17) glock 19 magazines were utilized for the study, 1 was empty, 1 was loaded with 1 round, another with 2, etc etc all the way up to fully loaded and left for 6 months. The remaining mag was used routinely in shooting exercises (loaded full and shot to empty, etc.) over the course of that same 6 months.

Findings were as follows:
empty to 12 rounds had negligible spring tension loss, 13 and 14 round mags had approx 5% loss, and the fully loaded mag lost about 10%. The mag which was used routinely lost almost 15% of spring tension.

Interesting huh?

Article courtesy of Joe LaSorsa

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