This question is commonly asked. I've read several of these discussions. The following is the best information I've found.
This was written by an engineer who participated at THR username Charlie Sherril.
Springs have a compression limit. If you compress a spring beyond that limit it will deform immediately and it will never work right again. If you compress a spring close to that limit it becomes susceptible to "creep strain" which will eventually cause it to fail.
The majority of gun manufacturers design their magazines so that even when fully loaded the load never approaches that compression limit. The problem is more with the off brand magazines like Pro Mags.
This is me
Magazines are expendable. Buy spares
Even though my research indicates that it's really not necessary, I keep enough magazines on hand for a given gun that I can put them through a long rotation cycle. 1 year loaded /5 years in the box. At that rate they should out last Methusala.
The question of "fatigue limits" and "creep strain" (which apparently is only a factor at 4X the melting point (Degrees Kelvin) of a given metal) is discussed in detail here
Does static tension wear out a spring? | Physics Forums
This is another article about how springs are engineered with the concept of "set" already factored in. I assume that most magazine manufacturers order their springs just a little long to "allow for set". For instance the Shield magazines that are initially so hard to load. Once the owner loads the magazine the first time the springs take that "initial set" and after that even compressed solid they don't approach the compression limit
Compression Spring - Stress and Spring Set
Compression Spring Set
When a custom spring is supplied longer than specified to compensate for length loss when fully compressed in assembly by customer, this is referred to as “Allow for Set”. This is usually recommended for large quantity orders to reduce cost. When a compression spring is compressed and released, it is supposed to return to its original height and, on further compressions, the load at any given point should remain constant at least within the loadlimits specified. When a spring is made and then compressed the first time, if the stress in the wire is high enough at the point the spring is compressed to, the spring will not return to its original height (i.e., it will get shorter). This is referred to as "taking a set", or "setting". Once the spring is compressed the first time and takes this set, the spring will generally not take any significant additional set on subsequent compressions.
One way to deal with this problem is to make the spring initially a little bit too long and then compress the spring all the way to solid so that after the spring takes the initial set, it is now at the correct height to meet the loadrequirement. This is referred to as "presetting", "removing the set" or sometimes "scragging". Presetting is a labor intensive and relatively costly operation due to the amount of handling of the springs involved. In most cases, the customer will also handle each spring as the springs are assembled into the product. As part of this handling, the customer could press each spring and "remove the set" so that the spring will be stable and perform satisfactorily. Another alternative would be to assemble the spring as is and allow the first operation of the mechanism into which the springhas been assembled to "remove the set". In either case, in order for the spring to be correct after removing the set, the spring would have to be received by the customer in a condition longer than the final height.
