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I talked to a magpul dealer about this a few years ago.  He said the best thing to do is to only load 20 rounds in the magazine (if it's a 30 round mag.)or about 2/3 full for long term storage.  Less compression on the spring leads to longer life.  I tried storing a mag with 20 rounds in it for about a year and it worked fine when used.

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The dealer is an idiot.....Fully loaded does no harm.

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the dealer is not an idiot

his advice was sound, however he may not be 100% correct.

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How was the dealer's advice sound? Compression DOES NOT wear out springs! Cycling does. As long as mags are not loaded past their maximum capacity, there is no issue with storing them loaded.

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Compressing a spring doesn't fatigue it, cycling it does.  You can store a 30rd PMAG fully loaded indefinitely.  I had 75 PMAGS stored loaded for >1 year, brought them to a Carbine 2 class, and they all ran flawlessly during the 3 days.

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This "fact" has been part of AR15.com lore for some time.  It is technically accurate, but practically incorrect.

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How was the dealer's advice sound? Compression DOES NOT wear out springs! Cycling does. As long as mags are not loaded past their maximum capacity, there is no issue with storing them loaded.

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This "fact" has been part of AR15.com lore for some time.  It is technically accurate, but practically incorrect.  

Yes, cycling a sping is what leads to metalurgical "fatigue."  However, fatigue is not the only enemy.  Leaving a spring compressed will eventually make the spring's resting state closer to the compressed position.  The spring's metal may not be "fatigued" but it WILL reduce the spring's tension and potential energy.  

Real world example: Take a domestic V8 race engine with a high-lift cam.  It takes a heavy valve spring to keep up with the cam.  Those springs do not like to sit compressed.  If you leave them compressed for anysignificant  length of time, they are not metalurgically fatigued, but they do lose their tension.  That is why on high-lift engines in race applications, the crew will pull the rocker arms to allow the spring to be decompressed when the car is not in use.    

Think about it.  Take two identical springs.  Put them both next to each other, but set an encyclopedia on top of one of them.  Come back a year later, remove the encyclopedia.  Odd are, the spring that has been compressed for a year will not rebound to the full length of the spring that has been sitting uncompressed.  Uncompressed length MATTERS in terms of a spring's lifting strength––even if, metalurgically, the spring is not "fatigued."

Try it yourself while sitting at your desk.  Take two paper clips and equally open them both up like a spring.  Then set a book on one of them.  Take the book off tomorrow and let me know if the paper clip with the book on it is still as tall as the one without a book on it.  

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Paperclips are NOT springs.  Of course if you set a book on a paperclip it will bend and deform!

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Technically a paper clip that is opened up is a spring. But, I understand what you are saying. I freely and openly concende that a paper clip is not manufactured/heat treated to rebound to the "open" position (unlike the bent wire that makes a "spring" in an AR magazine).



However, the principle is the same. It just takes more time to deform a "dedicated spring" by leaving it compressed... but it will, eventually, deform.

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Thats kinda like saying the wind will enventually erode a mountain.  For practical purposes though, there's nothing to worry about.  Storing a good quality mag fully loaded for extended periods of time is very safe.

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I am not disagreeing. I have kept some of my glock and Pmags loaded for months at a time, and they all seem to work fine. What I am saying is that keeping a mag loaded WILL weaken the spring's tension over time. The question is, how long will it take before the spring ceases to reliably elevate rounds in the mag? I think the answer is dependent on the mag spring. People on here have proven that a year or two for a PMag is fine. But what about 5 years? 10 years? 20 years? We dont know.



I am simply trying to correct this (mis)conception that a compressed spring will not weaken. A static but compressed spring may not weaken metalurguically (as will a cycled spring), but it WILL weaken with regard to its overall uncompressed length––which has a direct effect on a spring's energy.

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so do we agree the guy behind the counter, is NOT an idiot?

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He did not go full retard.

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This "fact" has been part of AR15.com lore for some time.  It is technically accurate, but practically incorrect.  





Yes, cycling a sping is what leads to metalurgical "fatigue."  However, fatigue is not the only enemy.  Leaving a spring compressed will eventually make the spring's resting state closer to the compressed position.  The spring's metal may not be "fatigued" but it WILL reduce the spring's tension and potential energy.  





Real world example: Take a domestic V8 race engine with a high-lift cam.  It takes a heavy valve spring to keep up with the cam.  Those springs do not like to sit compressed.  If you leave them compressed for anysignificant  length of time, they are not metalurgically fatigued, but they do lose their tension.  That is why on high-lift engines in race applications, the crew will pull the rocker arms to allow the spring to be decompressed when the car is not in use.    





Think about it.  Take two identical springs.  Put them both next to each other, but set an encyclopedia on top of one of them.  Come back a year later, remove the encyclopedia.  Odd are, the spring that has been compressed for a year will not rebound to the full length of the spring that has been sitting uncompressed.  Uncompressed length MATTERS in terms of a spring's lifting strength––even if, metalurgically, the spring is not "fatigued."





Try it yourself while sitting at your desk.  Take two paper clips and equally open them both up like a spring.  Then set a book on one of them.  Take the book off tomorrow and let me know if the paper clip with the book on it is still as tall as the one without a book on it.  

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A properly designed spring will not be harmed by being in a "normal" compressed state.  Taking some random spring and putting an encyclopedia on it is likely to cause that spring to be pushed out of it's design limits, and yes, will cause failure.  A magazine spring should be designed for a range of motion from compressed with a full load, to having no rounds loaded.  If one was to over-compress, OR over extend that spring, you will damage it to some degree.

There is, of course, no guarantee that a spring is properly designed.  Not in a magazine, a race engine, or anything else.  Further, because of other limitations, some springs may intentionally sacrifice certain desired performance characteristics for others (size, weight,etc)  There is also no guarantee that any given spring won't suffer from any number of other potential failures due to damage, metal impurity, or other factors.

But assuming that magpul has selected an appropriate spring, and does appropriate testing (and we have no reason to believe the don't) The spring will not "wear out" by storing mags fully loaded.