An NRA Instructor who had been a mechanical engineer came up with an article on springs. Constant compression isn't want kills a spring, it is being "worked". If you have just a few magazines and are constantly loading and unloading them the springs have a "cycle life" that is how many times they can be compressed and uncompressed and still adequately function. Other causes of spring failure are rust and physical damage.

In metal AR magazines the reason one should keep 1-2 rounds down from the top as in 28 rounds in a 30 rounder, isn't for the good of the spring, it is the ability to reload the weapon when the bolt is forward. When most 30 round magazines are topped off there is almost no further spring compression as the follower is bottomed out. Try a speed tactical reload when the bolt is still forward and many times it is almost impossible to seat the magazine. This happened to several students in my last basic tactical rifle class who didn't listen and topped off all their mags. They would reload, fire a shot and then get a "click" as the mag had either fallen out of the gun or had dropped down just far enough that the round didn't strip off and into the chamber.

If you count out 30 rounds into a PMag you will find that they will often take an "extra" for a total of 31...leave it loaded like this with almost no compression and you end up with the same condition as fully loaded metal magazine.

My favorite magazine is the CProducts 20 round stainless steel magazine.

Bob