Key point is that they are used. The springs won't take a set if they are cycled, only if they are stored "long term" without exercise. I contend that maintenance of springy thingys includes exercise.
Not real sure on that, I know my spare 10/22 factory mag for instance stayed loaded for a few years without being used and fired fine when did use it and have stored it similarly more than once.
I too leave my mags loaded with no problems at all. I have 1911 and AR-15 mags that I leave with 6/28 rounds loaded. I have never had a failure yet. I know some people who claim on boards that it is a no-no but my experience has been that it doesn't affect them. May decrease the life of the spring eventually but as for dependability I've had no problems with them. I found some AR mags in storage just last month that had been loaded for at least 7 years. I took two out and shot them without a hiccup.
I have heard the reverse about 'Modern spring materials' I have taken a couple AK and a couple 1911 mags, stored them loaded for 5 years, and they work perfectly. I have a couple 1911 mags I loaded January of 2000 that I plan on testing in January 2020.
I can easily be persuaded that materials have been improved to the point that what you say is true. Still, my routine maintenance involves exercising the mags. Not doing so means I have to keep track of old vs. new. Not in my personality to go that far. Besides, exercise means MTing the magazine --.
I don't know alot about metals, (Don't know what Mting is for instance) But I HAVE read about and seen 'metal fatigue' where if you bend (Or compress) and then bend back (Or release) the metal (Whatever) you can snap it off/wear it out. -Sounds alot like your 'exercising the mags' I just shoot mine. JMHO. $0.02: I did the '5 year mag test' after reading about a 1911 that had been found with 3 mags, that had been in a chest loaded since the end of WW2. ...They worked also.
MTing = "emptying". Sorry about that. (I MT mine by shooting them dry also, which was sorta the way I was pointing. Pushing loaded rounds out raises blisters on my thumb.) Metals are strange things sometimes. Different alloys will behave differently. Some alloys "age" after heat treatment, some don't. Some will stand a lot of cycles within the elastic limit, some will fatigue sooner than others in the exact same duty cycle. In all cases, mag springs should be designed and made so that the elastic limit is never exceeded, and I'd bet a gonad that there are no reputable gun builders that aren't careful that does not happen. I don't know which alloy my springs are made of, and little interest in the research. And you are right, at least partially, about bending back and forth. That is normally referred to as "work hardening" by straining the metal beyond it's elastic limit. With normal steels, work hardening actually makes the steel stronger, that's why it takes more effort to make subsequent bends until it breaks. It breaks when the material gets brittle from over hardening. I have a sneaking suspicion that "to exercise mags or not" is a Ford vs. Chevy subject.
