As part of my ongoing project to 'fix' and upgrade a number of AR-15 rifles for some disabled students and friends of mine, I've had to diagnose feeding problems. Most of them have turned out to be related to magazines. I thought my students, and perhaps other readers as well, might benefit from a short primer on the subject. Note that most of what is said here refers to all magazines, whether for a handgun or a rifle.
The first and most important point is that magazines are relatively fragile items compared to the firearm itself. They're typically made of stamped metal or molded plastic (sometimes both, as in Glock magazines, where a steel liner is surrounded by a plastic shell). Some are stronger than others. For example, US milspec ('military specification') magazines for the M4 carbine and M16 rifle (also used by civilians for their AR-15's, of course, and often referred to as STANAG magazines) are made of relatively thin aluminum.
If they're dropped on the feed lips, the latter can deform; if they're trodden on, the magazine body will often dent or bend out of shape. Once any of those things have happened, the magazine will become unreliable and should be discarded. Of course, the military has ready access to as many magazines as it likes in its armories and supply depots, so this isn't a major factor for them. On the other hand, some aftermarket M4/M16/AR-15 magazines have established a legendary reputation for toughness and longevity. For civilians, who can't use taxpayer money to stock up on new magazines whenever we wish, such attributes are much more important than they are to the military.
Unfortunately, many shooters try to keep magazines in service even after they've become worn out or damaged, because they don't want to spend money on replacing them. If you're playing games with your guns, I guess that's your call - it's not like your life's riding on your weapons system. On the other hand, if you're trusting a firearm to preserve your life, you can't afford that attitude. Many instructors with whom I've trained have opined that a quality handgun should have at least 4-5 magazines of equally high quality, to ensure reliable feeding and function. I agree. As for a rifle, the US military's standard-issue MOLLE load-carrying equipment enables the individual soldier to carry up to eight 30-round magazines of 5.56x45mm ammunition, or 240 rounds if they're fully loaded (of which more later). That seems to me to be a perfectly good minimum - I say again, minimum - standard for how many AR-15 magazines a shooter should keep on hand: at least eight good-quality, fully functional, tested and proven magazines per weapon.
(For both handguns and long guns, I personally prefer to have more magazines on hand than the minimum quantities mentioned above. After all, what if you break or lose some magazines, or lend some to friends or relatives, only to find that resupply isn't immediately available? Even worse, what if new laws restrict the sale or possession of larger-capacity magazines? I'm sure many will choose not to obey such regulations, but they'll make it much more difficult to replace magazines as they wear out. Having a few extra units on hand is cheap insurance [along with enough ammunition to load them, of course]. Furthermore, I retain older magazines for training and practice, so as not to unnecessarily abuse my newer 'defensive' magazines. However, my background involves eighteen years in an environment of civil unrest and terrorism. I freely admit that this has made me just a tad more sensitive about such things than your average American civilian, so YMMV.)
Choose your magazines according to your personal requirements. I find the 30-round standard M4/M16/AR-15 magazine perfectly usable under most conditions, but when using a prone firing position they're a bit long, and can prevent me getting the rifle low enough to be comfortable (or less visible to potential opponents). For that reason I like to carry a few shorter 20-round magazines as well. On the other hand, some higher-capacity aftermarket magazines (such as the 40-round units available from several suppliers) are even taller than the 30-rounders, making it very difficult if not impossible to get comfortable or stay concealed when shooting from prone. I've tried very-high-capacity (and much more expensive) magazines like the twin-drum 100-round Beta Mag or Surefire 60- and 100-round units, but find they make normally light and handy firearms much heavier and more unwieldy. For a military unit wanting sustained full-auto firepower they make sense, but (IMHO) less so for civilian semi-auto applications. I therefore choose to stay with standard-capacity magazines.
Bear in mind that manufacturers usually improve their products on an incremental basis, and this can affect your magazines. For an example, see this discussion of Glock magazine follower generations. As Glock pistols have moved from 1st, to 2nd, to 3rd, to 4th generation weapons, so their magazine followers have been incrementally improved as well. If you have older magazines with very-early-generation followers, it might not be a bad idea to replace them with the current version (and new magazine springs while you're at it). The US military has also improved the followers of M4/M16 magazines over time, so that there are three generations currently in service (being replaced, of course, by the latest version). If you have older STANAG magazines for your AR-15, I strongly recommend upgrading the followers (I prefer Magpul's self-leveling follower for that purpose; I've found it very reliable).
There's also the question of which generation of magazines works with which generation of weapon. For example, Glock 4th-generation pistols have an ambidextrous magazine release catch that necessitates double notches on their magazines. Those mags will work just fine in earlier-generation Glock pistols, but the single-notch earlier-generation magazines will not work just fine in 4th-generation pistols. If you have Glock pistols from multiple generations, it's worth stocking only magazines that will work in all of them . . . because you don't want to find out the hard way, in an emergency, that this is a problem!
Sometimes aftermarket products offer significant improvements over original equipment. Unfortunately, their quality varies from excellent to execrable. I suggest you read the firearms forums to see who's buying (or not buying, or complaining about) which manufacturer or brand. In my own collection, for example, I have nothing at all from ProMag or Triple K, but lots of magazines, accessories and related parts from (among others) Magpul, Mec-Gar, TangoDown, Troy Industries and Wolff Gunsprings. (No, the companies I've named aren't paying or sponsoring me to mention their names.)
One can buy magazine upgrade or repair kits for many weapons; but bear in mind that they all use the existing mag body. If that's damaged or severely worn, it's often more cost-effective to replace the entire unit. Besides, if one shops carefully, new magazines can sometimes cost less than repairing old ones. For example, Troy Industries recently held an online Thanksgiving weekend sale in which a three-pack of its Battlemags (shown in Flat Dark Earth color on the left) was offered for $20 instead of the usual price of $42, with free shipping for orders over $100. That worked out to less than $7 per magazine - a real steal! (Yes, I got mine.) It's worth waiting for annual sales events like that, and stocking up while prices are more affordable.
A word of warning: unless you know and/or trust the seller, be careful about buying used magazines - particularly from gun show vendors. For a start, they usually want exorbitant prices for their magazines. Furthermore, they can't (or won't) tell you who previously used them, for how long, under what conditions, and with - or without - proper routine cleaning and maintenance. I'd rather buy new units, which can often be had for similar or even lower prices than many 'gun show commandos' want for their used stock.
As for keeping magazines loaded, this may be problematic in the medium to long term, for four reasons.
- Some weapons have trouble feeding the first or second round out of a fully-loaded magazine, because the pressure of the magazine spring forces the top round tightly against the bottom of the slide or bolt and prevents it being smoothly pulled from the magazine. This usually goes away after the first couple of rounds have been expended.
- If the magazine spring is of high quality, it shouldn't have any problem; but lower-quality springs can take a 'set' over time, where they don't regain their full range of compression and may exert insufficient pressure on the rounds for smooth, reliable feeding in a weapon, particularly the last few rounds in a magazine.
- If the walls of the magazine aren't sufficiently strong, they can bulge over time under the pressure of the spring on the ammunition in them, making them too 'fat' to fit into the magazine well. First-generation Glock magazines without a metal liner were particularly prone to this.
- Over time, magazine feed lips can begin to separate under the pressure exerted by the spring against a full load of ammunition. Magpul produced an innovative dust/impact cover for its second- and subsequent-generation PMAG's that reduced pressure on the feed lips, which was very useful; but I don't know of any similar offering from other manufacturers.
To avoid those problems I routinely download my magazines by 10%, rounding to the nearest (lower) whole number if necessary. A 30-round magazine will be loaded with 27 rounds; a 20-rounder with 18; a 15-rounder with 13; and so on. For long-term storage, where I anticipate leaving the magazine loaded indefinitely, I download by 20%, so that a 30-round mag will hold 24 rounds, a 20-rounder 16, a 15-rounder 12, etc. This has eliminated from my weapons and magazines the four problems I mentioned, and I therefore recommend such downloading to all my students. (Of course, if your weapons and magazines don't exhibit any of these problems, you're free to disregard those precautions. YMMV.)
Magazines are often subject to greater abuse than the gun in which they're used. For example, while training with instructors such as Massad Ayoob or at venues such as Thunder Ranch, I've dropped my magazines hundreds of times onto gravel-surfaced shooting ranges during rapid reloads. The impact has dislodged or even broken more than one floorplate; stones have scratched mag bodies; sometimes someone's trodden on a magazine during fire-and-movement drills; and dust, dirt, rain and mud have gotten inside them. Furthermore, as dozens, then hundreds, then thousands of rounds go through them, magazines accumulate the same shooting detritus (propellant residue, excess lubricant, etc.) that one finds inside a dirty firearm. However, few shooters (in my experience) clean and care for their magazines in the same way that they do their firearms.
I recommend that at least once per year (more often if they're hard-used or abused), magazines should be stripped down to their component parts and carefully checked. The bodies should be cleaned inside and out; springs checked for satisfactory performance; and followers and floorplates examined to be sure they haven't been damaged. Any worn parts should be replaced. I usually apply a film of dry lubricant inside the magazine body before reassembly. For magazines that will routinely be dropped during reloading, I also try to fit them with some sort of bumper pad on the floorplate to help absorb the impact and reduce the possibility of damage. (For example, most of my more-heavily-used AR-15 magazines wear Magpul assists.)
Finally, I find it useful to segregate magazines according to mission. For example, I have several older and/or lower-capacity Glock magazines reserved for training use only. I make it quicker and easier to visually identify them by fitting orange Tangodown floorplates to them. Also, if I want to reserve a set of magazines for a specific purpose or a specific weapon, I color-code them. I learned the habit from a friend who has three AR-15-type rifles and carbines. He's equipped each of them with stocks, grips and handguards in a different color (respectively black, flat dark earth and olive drab), and has a set of magazines in each of those colors reserved for each weapon. Another friend uses different-colored Magpul assists to differentiate between different groups of AR-15 magazines. Some people mark them with colored paint or tape, or allocate magazines from different manufacturers to specific weapons or purposes (such as loading them with a specific round). The possibilities are limited only by one's imagination (and real-world practicality, of course).
In short, if any of your magazine-fed firearms has feeding issues, check the magazines first. They're more often than not the key to the problem.