I was searching about this problem on google before asking here. I always heard this mentioned, was curious about the issue and wanted to know myself. I was reading another forum on the topic and found this article. Its a pretty good read.
What do you guys think? Does leaving a mag loaded for long periods causes to wear out? Or is it just a myth?
The whole article is copied and pasted below in case link doesnt work.
FindArticles > American Handgunner > May-June, 2003 > Article > Print friendly
Magazine spring madness: 'creep' to your 'elastic limit' to un-earth the urban legend of 'spring-set'
John S. Layman
The shooting sports are full of some of the most knowledgeable and capable people you'll meet anywhere. I've been impressed consistently with the abilities of those I meet at the range to diagnose and fix a gun problem with as little as some spray lube and a cotton swab. However, sometimes a myth will creep into the folklore.
The magazine spring myth has been around for many years and is growing in popularity. It goes something like this: "You should unload your magazines when they're not in use or the spring will weaken causing failures to feed." This has gone as far as shooting competitors actually unloading their magazines between stages to extend the life of their springs. A variant of this myth is: "You should never load a magazine to capacity and should always leave it one round short." What if you need that round some day?
Recently, I read an article in a gun magazine suggesting you rotate your magazines so the ones not in use can "recover and rest." The same author uses the phrase "spring-set" to describe weakness of a spring because it was compressed for a long time. Hogwash. There's nothing further from the truth. Springs don't care how long they're compressed and don't require rest, recreation or even a vacation from time to time.
Shameful Spring Benders
To put this one to rest, you have to understand creep. Creep is the slow flow of a non-ferric metal like copper, brass and lead under force. At temperatures outside of a furnace, steel doesn't have any appreciable creep. Under most conditions, steel flexes and then returns to its original shape. When pushed past its elastic limit, steel will bend and not return to its original shape. All designers of well-made magazines make sure the spring never approaches the elastic limit when the magazine is fully loaded. Honest. This means the spring will not weaken when the magazine is fully loaded -- not even over an extended time. Like 50 years. American Handgunner recently ran a story about a magazine full of .45 ACP that had been sitting since WWII and it ran just fine on the first try. So there you go.
Now that the light of truth is leaking out, lets talk about what is causing failures to feed. The only way to weaken a magazine spring is to flex it past its normal range (elastic limit). If this is happening, somebody is trying to overload a magazine or has "adjusted" it by bending the spring. Both of these could cause feed failures. Shame on you if you're a spring bender.
Carlton Nether, Customer Service for Beretta USA, tells us keeping a pistol magazine loaded for an extended period doesn't cause magazine spring failure, however, failures to feed can result. He says, "The ammo will 'roll' in the magazine. If the mags are kept loaded and moved around a lot -- say on a cop's belt -- the rolling action can, over time, cause creases in the cases. These creases can cause malfunctions. Also the top bullet will roll against the magazine lips and creasing can occur there as well. Just check old ammo that's been bouncing around in a magazine for a long time.
We tell police officers if they keep loaded magazines, take a few seconds to "cycle" the ammo. Periodically unload the mag and reload it in a different sequence. This movement will allow the bullets to be in different parts of the magazine and help eliminate creasing.
At STI, Dave Skinner, President and CEO says, "Personally, I rotate my 'under the bed' and 'under the seat' mags about every six months. I always empty them the 'fun' way and have never had a failure." Given what we learned above, this sounds like a good idea. Smith and Wesson customer service also says magazines can stay loaded indefinitely without hurting the spring.
As we add force onto a spring, it will displace the same amount for each amount of force we add. This is true until the spring passes a certain point called the elastic limit. Robert Hooke discovered this theory back in 1660. Hooke's Law states: "If the applied forces on a body are not too large, the deformations resulting are directly proportional to the forces producing them." Which means, in actual human being language, if we load a spring past its elastic limit, it permanently deforms. It still provides a force against the load but the force is no longer proportional. If this happens, when we unload the spring (such as when we empty a magazine that has been over-loaded) the spring never returns to a state where it can provide the same load for the same amount of displacement.
When a magazine manufacturer designs a spring, they plan for a preload. The spring is already compressed some in the magazine. On the curve below, this would be Point A. The spring compression would be designed to be below the Elastic Limit. When fully compressed, the spring would be at Point B. If the spring is ever compressed past the elastic limit, say to Point C, it won't ever behave the same. Like a recalcitrant lazy Uncle, it will have a lower spring force for each amount of displacement. On the drawing, the spring would now cycle between points D and E. This means that -- particularly with the last bullet or two -- the force pushing the bullet up would be less and lo-and-behold, a mis-feed might occur.
When somebody stretches your spring to "fix" your magazine, they are trying to get you back on the original curve. They may get pretty close, however, it's unlikely the spring will ever perform to its original design. The elastic limit is now shifted lower and your magazine spring may fail to perform fairly quickly.
Having said all this, if you have a magazine that isn't feeding right, what should you do? First, disassemble the magazine and clean it thoroughly. Then try it with new, factory ammunition in a freshly cleaned gun. This takes away some of the possible causes. If you are still having feed problems, send it back. Even the low cost, after-market magazine manufacturers will fix the problem at no cost to you other than shipping. If it's a magazine from the gun's manufacturer, let them troubleshoot and repair the problem. Otherwise, toss the mag. It's not worth risking your life to save a few bucks. And that's the truth.