Duane Liptak

Duane Liptak - Magpul Industries - AR-15 magazines have come a very long way since the introduction and adoption of the M16, as have many developments in chamber spec, gas pressures, materials, propellants, and design tweaks to the original operating system. Basic expectations for fighting rifle performance had traditionally been 500 mean rounds between stoppages (MRBS) for the system in clean environment TOP 03-02-045 testing. Even though that performance standard had been left by the wayside for several decades, these numbers have only recently come up for discussion to be changed. This is due to the prior standard being made laughably irrelevant by current performance capabilities. The most recent round of US government testing of the GEN M3 PMAG at Aberdeen resulted in well over 30,000 rounds between magazine-related stoppages and over 12,000 MRBS for any causes. Yes, that's 12 cases of ammunition between stoppages with a direct impingement system. The M2 PMAG isn't far behind that. So…how did we get to that point? What are the characteristics of a magazine that can achieve that level of reliability? I get asked that a lot, and so here's an attempt at explaining some of that. As a Magpul employee, I have some level of bias. I could write several thousand words on why everyone should use PMAGs, backed up with facts, and test data. This is because I absolutely believe in the product, and the data does speak for itself. But, in the interest of some level of objectivity, I'm going to back out a level and talk about the factors in making a good magazine in general, which will hopefully provide some useful takeaways. Again, I believe the solutions, geometries, materials, and characteristics of the PMAG are superior, or I wouldn't work here, and I won't try to hide that, so bear with me. Also, please note that this article isn't the whole story. As you might imagine, I, and more importantly, the full design and engineering department at Magpul, could "nerd out" on this stuff for days. However, if I do that here, I know it would make for a pretty dry read. So as you read, please understand each subsection in this article is just a scratch of the surface. I'll attempt to hit the highlights in a readable format while not getting into anything that we'd consider trade secrets. AR-15 Magazine Basics – What Does the Magazine Do, Exactly? The best place to start in defining magazine performance is understanding what we expect a magazine to "do." A magazine, to be useful, must: Present a supply of cartridges into feeding position, in all compatible platforms, that are in an "acceptable" orientation with respect to the chamber, feed ramps, and bolt face; must provide an acceptable amount of resistance to stripping the top cartridge forward by the bolt; must provide a stack rise time for the top round to be fed upwards into a stable feeding position, resting on the feed lips quickly enough to accommodate the fastest acceptable bolt speed of the host firearm—and must maintain consistency in these variables across the range of atmospheric and contaminant conditions, and after rough handling within variation that is acceptable to the host firearm. Easy, right? Yes, that's a mouthful, but still doesn't get to the "how" and to the practical criteria that we can demonstrate in choosing magazines, cutting through at least some of the "magazine nerd" stuff. So, let's break down how all that is supposed to work and put it in some practical terms. AR-15 Magazine Basics - Presentation and Stack. The presentation is king. Positioning the top round in the stack correctly to support the feeding stroke of the bolt carrier is where the "rubber meets the road" in magazine performance. Ideally, we want to present the round in a fashion that will allow it to travel as straight into the chamber as possible. Every surface the round touches and every angular deviation it must make in its path robs energy from the bolt carrier that might be helpful in closing and locking the action when fouled or dirty. While ideally we'd like to place the round directly inline with the chamber for best feeding, the presentation height has to be low enough to allow the bolt to travel rearward over the top of the stack to allow function. This is where feed ramps come in. They help guide the cartridge up to the appropriate height to get into the chamber—we just ideally want to impact the feed ramps as little as possible to preserve that energy. And so…we can just set our feed lips up to point the cartridge directly at the chamber, and all is well, right? Well…not exactly. If you set a presentation angle arbitrarily without a proper round stack/follower underneath providing enough stability to the top round, it will nosedive if unsupported in the front. Or, it will go bolt over base if unsupported in the rear. Slow or uneven stack rise (rounds pushed up by spring/follower) can also create these conditions if the round is not stable and supported on the feed lips by the time the bolt travels forward. Straight magazines for tapered rifle cartridges provide this support with an articulating or semi-articulating follower and biased magazine spring to give the desired support. The follower has to be able to "rock" a bit to accommodate proper support as the magazine empties, and the follower travels up. You can't have a TRUE "anti-tilt" follower in a straight magazine to feed tapered cartridges, or at least to feed them well. With a straight magazine, there are limits to how many cartridges can be accommodated in this fashion. The cartridges naturally want to form a curve when stacked. At a certain point, the articulating follower and spring method can't keep up, and the mag body also limits stack geometry. The feed lip presentation angle is also limited by this geometry. If you try to present at too high of an angle, it complicates the support problem, and at a certain point, the follower and spring can't keep up. Eugene Stoner was a damn smart man. With stamping technology and straight magazine bodies, Stoner settled on a presentation angle that relied a good deal on the feed ramps, but that could be supported by the follower and spring. This worked fairly well. But what if we want more cartridges in the magazine? For the best function, the answer is to curve the magazine. Due to the combination of straight mag well section in the M16 and limitations of stamping at the time, the USGI 30-rounder used a "dogleg" curve. This accommodates the curve of the cartridge stack far better than a straight magazine, but it's a compromise. We still can't have a 100% anti-tilt follower. Some follower articulation is required to navigate the changing curve radius inside the magazine, but we can get closer. There are also inconsistencies in stack support created by the follower navigating the curve, which can manifest in an increased incidence of stoppages between rounds 14 and 24 in the stack or thereabouts. This is especially true when the magazine is dirty. There are also still limitations on the presentation angle that can be supported. Ideally, we would want something very close to a "constant curve," described by the radius of a circle created by the tapered rifle cartridges. This allows a true anti-tilt follower and permits support of the round stack at higher presentation angles by properly arranging the stack. Polymer magazine bodies generally allow the closest approximation to a proper, "perfect" round stack in the AR platform. We also need to have enough space laterally for the rounds to stack perfectly in that orientation. This means the rounds nest together in an alternating fashion, where each round contacts four other rounds surrounding it. This nesting allows rounds to be pushed straight up without tangential pressures (rounds that don't nest properly are pushed outward as they are pushed upwards), increasing the drag of the round stack on the magazine walls. We also want some room for ribs. Ideally, rounds should only contact the magazine walls at a few points (ribs) to minimize drag and provide room for grit and fouling to pass. This is why trying to shoe-horn rounds with a larger case head diameter than 5.56/.223 into the AR mag well generally doesn't work that well. This forces a compromised lateral stack, increasing pressures on the side walls. Raising spring pressure to apply more vertical force to overcome that also increases lateral pressures, increasing drag. Then, we're off to the races of unintended consequences and a parade of stoppages created by poor stack rise. "From the early magazine programs, in 2004: USGI magazine with Magpul Enhanced Self-Leveling Follower, L-Plate, and modified for a MagLevel window. After the introduction of the Magpul, the founders started looking at ways to improve the USGI magazine itself, and all of these efforts followed. This also exposed the limitations of the USGI magazine that could not be solved with accessories, and kicked off research into what would become the PMAG." AR-15 Magazine Basics - Pushing the stack. Once properly arranged, we have to apply force to the round stack to continue putting the top round in the appropriate place. This brings us to the spring. Spring rate, design, materials, treatment, and finishing is a broad, complicated subject. Suffice to say that we have to design a corrosion-resistant spring with an appropriate coil angle to apply the right bias to the follower to work with our follower design and stack geometry, and our follower has to transfer that spring force consistently as the stack rises/depletes with the operation of the firearm. If you don't properly design and constrain the spring path, you can get all sorts of weird spring force changes throughout stack travel. This destroys our chances for consistency in presentation, regardless of geometry. We also have to, or at least should design the spring to allow full capacity without reaching a "solid" state, where all...