Wear durability simply refers to how quickly parts of a mechanical system wear out. All else being equal, a gun made with high-quality materials will break less often than one made with low-quality materials.
Fortunately, most modern firearms have excellent wear durability. However, even with modern materials and modern quality control, gun parts still wear out. This article roughly addresses the concepts of optimizing weapon reliability from a mechanical wear perspective.
In basic terms, the more a part is used the more it will wear. The more it wears, the more its shape changes. This change can present itself as a loose trigger pin, or as chamber throat erosion that results in decreased accuracy.
Wear is detrimental to reliability. If we run any gun long enough it will eventually fail as a result of wear. The failure may be a broken spring, a cracked firing pin, or a cracked frame, but with enough rounds any gun will break. The likelihood of a wear-out failure increases with the number of rounds we put downrange.
It is important to keep in mind that not all wear is visible. Springs, barrel lugs, and other gun parts can all experience material fatigue that is not visible to the human eye. This fatigue can present in several different forms. Sometimes it is microscopic cracks that can suddenly grow into large cracks at the most inopportune times. Sometimes it presents as overly work-hardened metal that has become brittle and cracks into pieces. Whatever the form the results are the same: your weapon goes down with a hard malfunction.
If we are concerned about weapon reliability then it is clear that we need to address the issue of wear. If wear was the only thing that affected reliability then the simplest solution would be to always use a brand new gun. However, even if we ignore the cost of replacing the gun with every use, there are still other problems.
Most people have heard that guns need to fire a few hundreds rounds before they are fully 'broken in..' As it turns out, this also applies when replacing major parts such as the frame, barrel, or slide. In these cases, the first few hundred rounds serve to 'lap' the components to each other so they fit together more smoothly. Also, any residual machining burs or sharp edges may be worn down by these first rounds.
One additional complexity is that parts with undetected manufacturing or material flaws tend to fail right away rather than later in their expected lifespan. This is sometimes referred to as 'infant mortality.'
For example: Imagine there is a type of retaining pin that normally has a lifespan of about 15,000 rounds. If the metal rod stock the pin was made from has a defect, then the pin is more likely to fail between 0-5,000 rounds than between 5,001-10,000. In other words, if there is something wrong with a part it is likely to show up right away.
There are competing forces when it comes to firearm reliability. The first is 'infant mortality' and the second is 'wear-out.' In practical terms these competing forces generally result in firearms being most reliable somewhere between 20% and 70% of the way through their expected lifespan.
Of course, it is never that easy. If you replace 1 of 2 key parts, how do you figure out where you are in the overall lifespan? Obviously, the math gets complicated rather quickly. So how is one supposed to keep track of all this? The short answer is performing regular and complete maintenance.
If I had to create a rule of thumb, I would say that ideally no part in a fighting firearm should be at less than 20% of its expected service life and no part should be at more than 70% of its expected service life. Let's call it the '20/70 - shakedown/rebuild rule.'
When you repair a gun, replace all of the wear parts such as springs, extractor, ejector, etc. Also, it is important to keep in mind that barrel and trigger surfaces are wear points as well and should be checked. Once the weapon is fully rebuilt use it for training until you reach about the 20% point. This break-in period will allow you to shake down the weapon and ensure that there are no major function issues. At this point the weapon can be considered real-world deployable. You can continue using the weapon until it gets to around 70% then rebuild it again. Don't wait for it to break.
What about all those still working parts left over from the rebuilds? Use them in ‘Training Only’ guns. If they break during training, it is just good practice recognizing and dealing with weapon failures.
The next item that begs attention is that not all parts wear at the same rate, so not all parts have the same life expectancy. We have to sort out what part or parts we are referring to when we say 20% of expected lifespan. This is where the practical has to take priority over the theoretical. It’s not practical to keep every part in the gun, at the exact same state of wear, all the time. Again, we will need to go with a rule-of-thumb.
Most modern firearms can be expected to function for at least 10,000 rounds before any parts have ‘wear-out’ issues. So for many guns, it is reasonable to run 2,000 rounds through them during training to make sure they are solid and running smoothly. So continuing with the 10,000 round example we should rebuild the weapon somewhere around 7,000 rounds. This lets us deploy the gun over the 5,000 round period in which it is most reliable.
The 10,000 round numbers are simply an example. These numbers certainly are not correct for every weapon. Machine guns tend to remain reliable through higher expected round counts, whereas, the barrel on a sniper rifle is likely to become ‘shot-out’ by 5000 rounds. So for a sniper rifle the burn-in period may be 1000 rounds and the rebuild point may be 3500. Every tool is different. There isn’t one number that will work for every gun.
The key is to understand the general principle that mechanical devices tend to be more reliable near the middle of their expected lifespan. If you have the option of optimizing so that deployed weapons are in that 20% - 70% wear range, then you should avail yourself of the opportunity.