So, the subject of downloading magazines has come up again recently.
This is the practice, that some people swear by, of not loading your magazines up to full capacity. In their estimation, this extends the life of the magazines spring, and enhances reliability in feeding.
... except generally speaking, for most weapons and most magazines, it doesn't.
There are exactly two common modern weapons , where consistently downloading magazines has proven to be a good idea.
The first is the M16/AR15; in particular the 30rd magazines for it. Theres three reasons for this:
1. AR mags are mostly made of lightweight aluminum, that may or may not be particularly structurally rigid. If you load all 30 rounds in some 30rd magazines, the magazine body, baseplate, and feed lips can slightly distort, which can cause binding, feed errors, and in extreme cases can cause the mag catch to fail, or the baseplate to pop off the bottom of the mag if hit on a hard surface.
2. AR feed lips often have distortions or burrs. If the 30rd magazines are fully loaded, and have a newish spring, the first few rounds from the mag can be pressed firmly into the distortions or burrs and cause binding. Heck even with completely pristine feed lips, if you have an overly strong mag spring, and an understrength operating spring, the rounds can bind.
3. If you have an overly strong mag spring, it can be difficult to ram an AR mag home on a closed bolt and have it lock in place. This is bad practice anyway, you should generally load an AR mag with the bolt held open; but sometimes you don't have time to check and then operate the mag catch; and sometimes you want to do a tactical reload (where you replace a partially empty mag with a full one, while keeping a live round in the chamber).
Because of this, I will often download my 30rd AR mags by 1-3 rounds; depending on tension and the rigidity of the mag body. These issues generally don't occur with 5 or 10 round magazines, but sometimes can with 20rd mags. It's a matter of judging through experience; or doing it no matter what just to be safe.
The second weapon I will routinely download the mags for, is any Glock auto pistol. I do this, because the Glock mags are shipped with a somewhat over-strength spring, that has yet to take its permanent set. This makes the last round (or two) very difficult to insert, and in fact can cause the magazines to bulge just a little bit (especially the ones without the metal liner, often called "non-drop-free").
Much as with the AR, this tight spring can cause the top round, or even two rounds, to bind and not feed, and can cause the magazine to distort causing misfeeds.
After the magazine has been fired through a few dozen times, the spring will usually have taken it's permanent set, and will be easier to load, and feed from. Sometimes it's still tight enough that I'll keep downloading a round, but usually not.
The upside to all this of course is that Glocks always feed the last round properly, and the mag springs last longer; something that cannot always be said of other magazines.
Oh, and quite a few folks report that 8 round 1911 magazines can also detension if left fully loaded. I've seen it happen with lower quality magazines, but not with Wilson or McCormicks powermags.
The problem here isn't one of poor springs, or poor mag design, it's an issue with going beyond the original design parameters. The 1911 magazine was originally designed to hold 7 rounds, and by necessity the magazine follower and spring have to be compressed down a fair bit more to hold that 8th round. As a result they use a slightly shorter spring with slightly lower diameter wire, and a thinner follower. This leaves less tolerance for errors, less clearance overall, and a greater potential for a spring to be be over-compressed, r to take a set with just a little but less pressure than needed to feed the last round properly or lock the slide back.
Your mileage may vary (as I said, I've never seen the problem with Wilsons, but many others have), but if you do have an issue, buy a Wolff replacement spring, and download to the original design capacity of 7 rounds, and you'll be just fine.
The biggest reliability problem with semi-automatic pistols and rifles in general, is in fact, their magazines. There is a difficult balance between feeding the bullets fast enough, under enough tension, at the right angle and with the right release point; and having too much tension, releasing at the wrong angle, too early or too late.
To help alleviate these reliability issues, some people have the oddest ideas. I've seen people say bake your mags. I've seen people say freeze your mags. There are people who never lube them, and people who overlube them.
There are LOTS of folks out there who believe you shouldn't store magazines loaded, because it will cause springs to detension.
Well, again, for most modern well made firearms, this isn't necessary either.
Once a good quality spring has taken its initial set, it wont detension merely from being compressed; unless it is over compressed beyond the limits of its set. A properly designed magazine filled to capacity will not cause this over compression; and the spring for that magazine will have been designed to take it’s set to the appropriate length and tension.
What causes detensioning, is either overcompression as mentioned above, or the gradual loss of internal stresses due to working the metal through compression/decompression cycles (work hardening). Generally speaking this takes at least tens of thousands of cycles for even the cheapest of springs, though corrosion and heat cycling can accelerate this process; and poor design and manufacturing almost certainly will.
Springs can be designed with hundreds of millions, or billions, of cycles life expectancy (like valve springs in cars for example). Of course firearms magazine springs aren't nearly as tough as cars valve springs, but it is reasonable to expect that with a properly designed magazine, manufactured to appropriate standards, a magazine spring should last for thousands of cycles.
On thing you should NEVER do though, is stretch a spring beyond its set point. If you do, you are going to instantly cut the life of that spring; and in fact you're making the issue you're trying to fix (too short a set) worse, because the spring will now re-set even shorter after a few cycles.
Once a spring has taken its set, that's the longest it's ever going to be. If that isn't long enough, you need a different spring, it's that simple.
So anyway, leaving the mag under constant partial tension won't cause a problem; presuming the mag itself is properly designed, and a quality spring is used.
You should however cycle your magazines every few months, from loaded to unloaded, and back again, just to ensure that tension loss hasn’t occurred for some reason, and to ensure that stiction or corrosion aren’t causing the cartridges to bind in the magazine.
There are verified cases of auto pistol magazines, having been found; loaded and stored since WW1; that would feed and fire just fine.
The real problem, is that it's very easy to cheap out on magazines.
Even the factory magazines that come with many guns aren't of the highest quality (this is especially true of 1911 clones). Good sheetmetal that is both thin, and strong, isn't cheap. Good springs aren't cheap. Additionally, as I said, magazine design is a very tough balance of design and construction, to get things timed and tensioned exactly right. Good magazine designers, aren't cheap.
Thus, good mags aren't cheap.
If you are using a gun where the only magazines available are cheap, poorly made, or poorly designed; then yes, I can see an advantage to downloading your mags... but honestly I wouldn't trust such weapons unless I absolutely had to.
If I DO have a spring problem with a mag, I replace the spring with a Wolff, and generally speaking, never have to worry about it again. Wolff springs aren't cheap, but for the reliability and consistency they give you, they are cheap at twice the price. When you may be defending your life with something, paying for quality jsut seems like a good thought.
My primary carry guns are a custom 1911, and a customized HK USP compact. I only carry factory mags for my HK, even though they cost $50 a piece (and I have 5 of them). I only carry Wilson Combat 47d (with the low profile steel base pad), or Chip McCormick power mags for my 1911, even though THEY cost $30 a piece (and I have 8 of them... and really need a few more).
I would probably also use older factory Colt mags, or newer ones with Wolff springs in them (the springs in Colt mags since the late 80s have been iffy), if they weren't the same price or more as Wilsons or McCormicks because of those poor (and I mean that in every sense of the word... have yo seen some of those prices) misguided Colt fanatics who have to buy up every original mag they can.
Some factory Kimber and Springfield mags are either made by, or licensed from Chip McCormick. They are of the "Shooting Star" mag design, which is a bit different from the PowerMag, but also generally excellent. The problem with the factory licensed mags, is that you never really know what you're getting there to be honest. Sometimes they work fine, sometimes not, but just to be safe I only ever use them as practice mags.
The three other big players in aftermarket magazines are Metalform, Mec-Gar, and ProMag.
Metalform or Mec-Gar are the OEMs for almost all factory magazines not produced in house (that would be almost all factory magazines these days), and are generally of good quality; though they make their mags to the spec of the guns manufacturer, which sometimes means compromises in quality to meet a lower price. In fact in general the magazines they make for the aftermarket are better than the "factory" mags.
If there is a higher quality option, I will generally buy them for carry mags, but honestly that's just paranoia on my part; because I don't have any problem with either manufacturer as practice mags, or carry mags if there's no other option. In fact, Metalforms line of premium 1911 mags are every bit as good as Chip McCormicks; it's just their general run of OEM mags I don't care for, and again, that's because the factories tend to cheap out a bit on their mags.
ProMag on the other hand, are generally of poor quality. They also OEM for some manufacturers, but primarily they make what gunnies like to call "gun show mags". Cheap, poorly made magazines, often sold for less than half the cost of factory magazines, mostly at gun shows. Not only do they generally use a poorer quality of materials, but they also often slightly change the designs of the magazines they make, in order to make them cheaper to produce.
I won't even use a ProMag as a practice magazine; because I've just had far too many problems. I don't feel like wasting my money on a magazine that is just going to be crushed. The only exception, is when ProMag are the only option; which is unfortunately the case for quite a few guns that are no longer supported by their original manufacturer. Oh and some of the ProMag polymer mags are actually acceptable. They wouldn't be my first choice in an AR or SKS, but they seem to work reasonably well.
The reason why I carry only those mag types I listed, is because I know they are well designed, use quality components, and are reliable.
I have many thousands of rounds through both weapons, without a single misfeed due to the magazine. In fact, if I have a single malfunction caused by any magazine that isn't from a worn out spring; that mag gets permanently and prominently marked, and used only as a practice magazine. If it develops a pattern of malfunctions I destroy it so that it can never be used accidentally (or by some cheap bastard), potentially risking someones life.