Leaving an AR magazine loaded will damage it..RIGHT??
About 22 years ago I fully loaded a 30 round Okay Industries military surplus magazine and put it in my gun case with my old AR-15 Carbine, "just in case".
Today I took my new Colt M4 LE6920 to the range and decided the magazine had sat long enough if the spring was going to weaken or the lips spread, it would have happened by now.
It fired the entire 30 rounds perfectly with no problems at all. This leads me to believe that modern American made magazines don't deteriorate if left loaded long term.
The new Colt had only one stoppage, and that was a failure to fire. The round required some hard pulling to open the bolt, and there was no firing pin strike on the primer.
Re-chambered, it fired.
Since the round was an OLD reload, I wrote that one off to a slightly defective reload.