That is the question regarding loaded magazines. I routinely get this question asked in classes and emails and my response is pretty simple. First off, you should have two sets of magazines, a set for training and a set for duty. You have to understand the magazine is more often than not the weakest link in your firearm chain (firearm, ammunition, magazine and maintenance). A quality firearm will be well designed, but they will also have well designed magazines to feed them. I am more careful with my duty magazines; I avoid dropping them fully loaded, periodically clean them and rotate them at a certain interval.
I know there are several folks who will push a different agenda, but like I said earlier the magazine is the weak link. This is the main reason why I have two sets of magazines, because I don’t give a furry rat’s ass what I do with my training magazines, but regular abuse on a duty magazine will have a negative effect on the reliability. It shouldn’t take a rocket scientist to figure that out, but you would be surprised.
As for cleaning them, some duty magazines find themselves in the old “out of sight/out of mind” syndrome. We have seen some duty magazines pulled from magazine pouches on duty belts that were so covered in debris it would have been a true testament to the reliability of the firearm had they functioned flawlessly. I have seen a hollow point cavity literally filled to the rim with crap.
I will rotate my duty ammunition and magazines on a set interval, which is usually twice a year. Your mileage may vary, but that is what I have found to be good for my intentions. However, with ammunition being difficult to come by some folks may want to abstain from actually firing their duty ammunition. Twice a year, I will fire my entire load-out of duty ammunition during a professional development range session. Before I leave the range, I will clean the old duty magazines then I will take fresh duty magazines and load them with new duty ammunition until the next interval.
Some will complain about spring set and having to unload their magazines more often. If the magazine was designed correctly then it was designed to take the load. In other words you don’t have to unload it to save the spring. Certainly avoid tampering or bending any springs, as that WILL affect the reliability of the spring. If you force a spring past its elastic limit, which under normal loads you don’t have to worry about, the spring will still apply force, just not the same force and that is where reliability can become a problem.
I once found a loaded magazine in one of my old kit bags that had to have been there for over 15 plus years. It fired flawlessly, so I’m not too worried about the reliability of a good magazine loaded over extended periods. What I worry about is the exposure to the environments and normal wear and tear on a daily basis and that is the main reason why I download them.