I have kept some PMAGS loaded in truck all summer. I live in New Hampshire. Temps have hit near 100 this summer and are in the -20+range on my way to work in the winter... none of these mags had issues. I have some metal USGI mags from BCM. I feel much better about storing these loaded than plastic mags. Still... I have had Pmags loaded for 2+ years subjected 100+ degree temp swings without issues....