The question recently arose, in discussion with my wife and some other writers, about whether it was safe to keep one's firearm magazines loaded for extended periods (we're talking years rather than months). There was considerable disagreement about it.
To begin with, let's clarify the discussion a little. Here's a video analysis of how magazine springs work, and what's within their design specifications. It looks pretty accurate, judging by my knowledge of the field (which is that of an educated and experienced firearm user, not an engineer, I hasten to add). I don't fully agree with his conclusion, though - more about that later.
During my military service, I learned a lot from a few former US servicemen. After the Vietnam War ended, they didn't want to go back to their homes, because of attitudes towards them in civilian society. Several joined the Rhodesian and South African armed forces, and I'm glad they did, because what they taught helped keep me alive during some rather difficult times. They based their comments about magazines on what they'd experienced in Vietnam. This was, of course, with the first iteration of the M-16 rifle and its 20-round straight-sided magazines, neither of which were the world's most reliable at first.
They told us that the magazine spring had to be in good condition and free of contaminants like sand, dust or mud, which might cause friction and binding against the sides of the magazine. Since Africa had more than enough contaminants to go around, that made a lot of sense to us. They also pointed out that when the magazine was loaded to full capacity, the top round might protrude slightly higher than normal, due to the pressure on it from the rounds beneath it. That could cause it to drag against the internal components of a rifle during the autoloading process, and potentially cause a misfeed. Finally, if the magazine spring was weak or defective, it might not feed correctly under the stress and pressure of a full load. For all those reasons, they had learned to download their M16 magazines by 10%, and recommended that we do likewise. If we had a 20-round magazine, load it with 2 rounds less (i.e. 18); if a 30-round, load with 27; if a 35-round, load with 31 (ten per cent of max capacity is 3.5, which is then rounded up to the nearest whole number).
I've followed those guidelines ever since. During my military service, carrying first an R1 rifle (a South African licensed version of the FN-FAL) and later the R4 (a ditto copy of the Israeli Galil), I always downloaded by 10%. Even today, out of long habit, for any magazine - long gun or handgun - holding 10 or more rounds, I'll download by 10% for carry purposes and short-term storage. In my Glock 17, for example, I load the 17-round magazines with 15 rounds (10% of capacity is 1.7 rounds, which rounds up to 2), plus one round in the chamber.
The former US servicemen went further when it came to long-term storage of loaded magazines. Remember, their experience had been largely with the first-generation M16 magazine, the spring of which was not very strong, and could take a "set" if kept in a tightly compressed condition. Their views were strongly ratified by several Rhodesian and South African servicemen of my acquaintance, who reported similar problems with their R1 20-round magazines. For storage longer than a few weeks, our instructors recommended downloading by 15% to 20%, depending on the quality of the magazines concerned. Thus, a 20-round R1 mag could be safely stored for months or years with 16 rounds in it, and a 35-round R4 mag with 28 rounds. Again, I've followed those guidelines ever since. Some claim that modern magazine springs are better than older ones, making downloading unnecessary. I'm not so sure. I work on the principle that Murphy is out to get me, and I have no way of telling how good my magazine springs may be - particularly if I don't know when or where they were made. I'll continue to download mine, thank you very much.
I keep quite a few loaded rifle magazines on hand, not because I'm paranoid, but because it saves a lot of time. If you've got to load 10 magazines at the shooting range with up to 30 rounds apiece for a practice session, that's an expensive process when you're paying by the hour! I'd rather take 10 loaded magazines with me, and be good to go as soon as I reach my firing position. I can load them at my leisure when I get home, even if I do just one every day or two. After all, if I have to store both magazines and ammo, why not store them together like that?
I'm also a firm believer in keeping a few magazines on hand for "social use", loaded with defensive ammunition, just in case. In my younger days, I found myself in too many situations where there was no time to load magazines. Sometimes the fight comes to you, whether you're ready or not. (Ask anyone who's had to survive a home invasion, or run into urban unrest while driving.) Also, if some sort of natural disaster threatens your home - for example, a tornado, a hurricane, etc. - and you need to leave in a hurry, the press of other last-minute details means you may not have time to load magazines for security while you travel. I keep a 'battle belt' and/or chest rig carrying loaded rifle and handgun magazines, ready for use if needed, near where I store my firearms. All the magazines are downloaded as described above, but hold enough ammo to take care of initial needs; and I'll have spare rounds to top them up, if I have time to do so. If I need a gun in a hurry, all I have to do is slap in a magazine, cycle the action, and I'm ready to go.
You don't have to get complicated or expensive about the battle belt or chest rig. Remember the KISS principle. The more gadgets and gizmos you have on it, the more likely you are to break something or get tangled up in it. In Africa, as a civilian, I used the old Chinese Type 56 or Type 81 chest rigs, and modified them to suit my needs. I'd run into them often enough on African battlefields to know they're pretty tough, and they don't cost much even today. You can cut off one or more of the pouches if necessary, and replace them with a different style of pouch - e.g. a cellphone holder, a small IFAK (medical kit), etc. - for greater versatility. A simple chest rig like that, added to a belt holding your pistol holster, handgun magazine carrier and a flashlight, can be very useful.
Anyway, those are my thoughts on keeping loaded magazines on hand. What has your experience been? Do you do things differently? Let us know in Comments, so we can learn from each other.