Re: Leaving magazines loaded
Couple engineering points to consider on this wive's tale. And just to say it up front, I used to be guilty of swapping ammo all the time in and out of SD weapon mags "just in case" it was true. I've since changed my mind for the reasons mentioned below.
1) Unlike AL, Steel does not "creep" and springs are made from steel
2) Steel does fatigue
Therefore:
I've come to the conclusion from leaving things loaded for a couple of years that there's no issue on leaving a mag loaded for many years. You do more damage to constantly loading and unloading it.
That being said, in a duty weapon exposed to elements I would still suggest disassembly of the magazine every few months to a year in order to clean out any debris, dust, dirt, etc that has collected. This is more common to mag failures than anything I have seen.
On USGI issue M16/AR15 mags the sharp corner in the aft edge of the magazine where the shell base sits is the failure point on the old mags my dad has had to throw away. They crack in the corner and soon have little ability to properly hold rounds.
This is part of the reason why the Pmags have a clip on the top where puts light pressure on the rounds. It removes the load from the top of the little corners and relieves that stress. This further compresses the mag spring, and the Pmag is about the best damn thing since sliced bread for that weapon system. This should show you MagPul's position on leaving a loaded magazine stored for years.
Finally, I never load a mag to full capacity. My G29 holds 10+1, I carry it with 9+1 in it. The AR mags that get kept loaded are 30 rounders that I only keep 28 in at a time.
Any time that I've had a mag failure with a clean mag at the range has been because I tried stuffing the max number of rounds in it and it jammed on me. I've never had such a mag failure partway through a magazine.
My feeling is I'd rather have 28 rounds come out every time and have to change mags than have "BANG...shitshitshitshit"