Form, Fit, Function: A Firearms Design Science

By Paul Evancoe

Aesthetics, ergonomics and accuracy certainly play a role in firearms design, but those intangible qualities usually verbalized as “the gun just feels right” or “it’s a sweet shooter” are major contributors to making it your favorite.

Confidence in a particular gun’s performance may also be a reason to favor it. These perceptions are not coincidental, rather, they are the result of the weapon designer’s careful consideration of form, fit and function (F3) engineered into a functional firearms design.

What exactly does form, fit and function mean? F3 is most commonly used in relation to the design of an object or when considering if a particular feature is value added or not.

• Form describes the physical characteristics of the gun. It includes things like shape, feel, weight, color, material, etc.
• Fit is short for “fits intended application.” Alternatively, it may also reference whether the physical dimensions of a part fit into the product it was designed to go into. Fit is also concerned with system interoperability and parts interchangeability bwetween like systems.
• Function is about what the product is intended to do and actually does–its intended design function compared to its operational suitability to perform that function.

Form, fit and function is also a good litmus test to check whether a manufacturing process step is value added. The proper tightening of a screw that holds parts together, replacing metal with polymer or lubricious coating of specific high friction parts are examples of steps that change form, fit or function for better or worse, depending upon the design. Most customers value F3, so it follows that a step improving the overall F3 would be something those customers would be willing to pay for–but maybe not, especially if the F3 step is negligible to customer perception as value added.

If the step does not positively change the F3s, it is likely a non-value added step that should be eliminated if possible. It may still be necessary, but it is not something the customer might accept in terms of increasing his cost. A manufacturer can easily price himself out of a market without realizing he’s done so by offering a 10% performance increase that costs 40% more than a comparative firearm or part(s) replacement solution.

Whether during its initial design inception, or when changes occur throughout the product’s life cycle, most engineers pay close attention to the form, fit and function in product design. Successful companies like Glock, Magpul and GEMTECH, with a strong continuous improvement culture, tend to have more product design evolution (generational improvement) than others, as each person in the company tends to assume some of the roles traditionally filled by manufacturing engineers. These companies have a culture that invites feedback recommendations for product and process changes in order to continuously make their products better, and as a result, their products reflect superior F3 quality and popularity.

Product evolution can be readily seen in the generational product upgrades some firearms manufacturers offer on a somewhat regular basis, but change is usually expensive and time consuming. It can require new molds or modification of existing molds, new materials, new tooling, or part and product redesign. As a rule, even if a change makes a production process easier or cheaper, it must not change the positive perception of the F3s in the customer’s eyes.

There are some classic firearms designs that exemplify form, fit and function: Colt’s Peacemaker revolver, Colt 1911 .45ACP, Browning HiPower, M1 Garand, Uzi MP-5 and today’s Glock product line and AR-based designs, including FN’s FNX .45 Tactical pistol, to name a few. Even though these guns span a range of two centuries and are very different in form, fit and function, they have all enjoyed continuous popularity. Why? Short answer: they look good, they feel good, and they do what they were designed to do.

Today’s FNX .45 Tactical pistol, for example, was specifically designed as a high capacity, high reliability, low maintenance, fully ambidextrous combat pistol. Its materials are primarily stainless steel and polymer for corrosion resistance, weight reduction and easy cleaning (dishwasher safe). It’s ruggedly designed to withstand the rigors of combat. With fully ambidextrous operation of all levers, catches and release buttons, it feels equally good in either the left or right hand. It further exceeds its advertised performance and reliability claims. It reflects the epitome of F3 engineering for a combat handgun. Would it be suitable for concealed carry? No, but that is not its design function.

Take a moment and think like a firearms design engineer who is either creating a new firearm or making design upgrades to an existing firearm. Consider how form, fit and function might be incorporated into a firearm’s design and what those attributes will mean to the customer. If you’re designing a pistol magazine, for example, it might be easier and cheaper to permanently weld the magazine baseplate onto the magazine body. However, for the typical gun owner without an ultrasonic cleaning machine and an air compressor, that would make cleaning the magazine internally, along with its spring and follower, much more difficult, if not impossible. So proper magazine cleaning would likely not be accomplished and ultimately result over a period of time in round feed or extraction malfunctions from an internally dirty or rusted magazine.

Therefore, the “fit” of the magazine into the needs of the owner, specifically low maintenance requirements, is weighted against the reliability of the entire pistol operating system throughout its lifetime.

As a matter of fact, this very situation is not uncommon throughout the firearms industry. The solution is evidenced by the range of aftermarket magazines offered that often work more reliably than factory magazines because of their design. Magpul’s Gen2 and Gen 3 AR magazines are a prime example of superior aftermarket designs that are perfectly aligned with AR-style weapons’ F3.

The aftermarket can also kill a well-designed weapon’s reputation. All firearms are designed with a total rounds-fired life expectancy in mind. Critical parts are replaced on a rounds-fired basis. Barrels and bolt carrier groups, for example, might be replaced at 15,000 rounds; the upper receiver may be replaced at 20,000 rounds; the trigger assembly at 25,000 rounds and so on and so on. All firearms sold to the U.S. government have technical data packages (TDPs) that contain detailed design and manufacturing blueprints of every part and screw, along with each part’s specifications and tolerances. Tolerances are always expressed in plus-minus terms (maximum limit and minimum limit for part fit).

In most cases the government doesn’t buy replacement parts from the gun’s original manufacturer or hire the original manufacturer to rebuild its own guns. Rather, the government sets the contracts for spare parts and depot-level maintenance aside for small and disadvantaged businesses.

There may well be several different manufacturers all supplying the same spare parts for a particular firearm. These disparately manufactured part lots are put into a spare parts bin without lot concern. This means the plus/minus tolerances are mixed together and become homogenous to the armorer who installs them.

Take Fabrique Nationale de Herstal’s (FNH) M249 Squad Automatic Weapon (M-249SAW), for example. The M-249SAW is an automatic fire light assault machinegun chambered in 5.56 NATO. It has been a mainstay across the U.S. military for over four decades. It is an F3 masterpiece of design; however, since its mainstream use in the Gulf Wars beginning in the early 1990s, it has gained a field reputation as unreliable. Why?

The M-249s that have been in service have been used hard. They have all received depot-level maintenance numerous times. They’ve been rebuilt by various vendors from the frame up with spare parts. Here’s the problem. While the spare parts used in these guns may fall within the plus/minus tolerance required in the manufacturing technical data package, the depot installers don’t check to see if they are randomly installing parts that are mostly plus tolerances or minimum tolerances. This means if several full-limit, plus tolerance parts are installed in conjunction with others of the same tolerance (and expected to function properly) the group will be loose fitting and sloppy. The same is true if minimum tolerance parts happen to be installed in conjunction with other minimum tolerance parts. The group will be exceptionally tight.

Now consider the mechanical consequences throughout the operating system in a rebuilt M-249 machinegun, resulting from the heat generated by extensive automatic fire in a combat scenario. The minimum tolerance tight parts wear excessively, and the loose fitting parts become even looser from heat expansion. Both lead to gun failure. Thus, the gun gets a bad reputation, not through any design deficiency, but because of the lack of F3 attention to detail in the spare parts being used and a complete discounting of F3 during depot-level maintenance. Ironically, none of which the original gun manufacturer has any control over. This is a lesson in how not to conduct maintenance—yet that’s exactly how it’s done every day. Without manufacturer’s configuration control and part quality control, it is difficult, if not impossible, to continue the F3 process throughout the firearm’s life expectancy.

There are additional hidden pitfalls. Some engineers don’t fully understand what the gun’s performance requirement parameters mean operationally, and they get it wrong. That’s why product managers play such a critical role in lifecycle and configuration management. They must understand F3 and the consequences of discounting its importance over the lifetime of the firearm.

Finally, and to be fair, there are occasions when a firearms manufacturer, without any concern or understanding of F3, gets lucky and comes up with a great gun. This is most often evidenced in the many aftermarket ARs and AR replacement parts on the market today. Many of the very best ARs are a kluge of parts manufactured by a multitude of aftermarket providers and assembled as one brand by a manufacturer who may only produce an upper or lower of their own. These ARs offer a vast range of barrels, grips, rails, stocks, catches, levers and releases. They are assembled without concern for tolerances and some, by sheer luck, perform very well. Others don’t deserve the recycle bin.

The lesson here is to make very careful choices on the parts you replace in your firearms. They may have great form and look very aggressive and “cool,” but they miss the fit and function requirements to work properly as a part of the overall firearm. Correspondingly, just because a preplacement part fits, doesn’t mean it will function properly and perform its purpose in life. This is also true (if not especially true) for accessories like slings, grips, rails, sights, optics, lights, etc. Granted, “tricking out” your favorite firearm is fun (as well as expensive). The vast majority of the stuff people put on their guns may look impressive, but it actually detracts from their firearm’s overall form, fit and function and hence operational reliability. Now you know. Choose wisely.

This article first appeared in Small Arms Review V21N4 (May 2017)
and was posted online on March 17, 2017


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