"Unsealed, sliding, simple machines operating under a narrow window of energy and friction, in very harsh environments - that's guns. You have to protect that window of energy and friction - that's your lifeline."
Educate yourself - The Machines We Call Guns
Guns are unsealed machines - environmental contaminants and combustion gasses add friction and can damage lubricant. No seals also puts a premium on lubricant staying where you put it.
From the lubrication perspective, some of the key design parameters of guns are obvious - there is no reservoir of lubricant in these machines, and no system to circulate or filter lubricant. Their friction surfaces are unsealed - against the outside environment, against combustion gasses, or in keeping volumes of lubricant in place. And they operate globally, in every extreme.
Each of these factors has tremendous consequence for lubrication design. We cover this in more detail later, but at a minimum, these operational parameters require a lubricant to stay where it is placed, with maximum capacity to keep contaminants away from load-bearing, friction surfaces. It must also be hydrolytically stable, meaning that it shouldn't degrade in the presence of water, or readily suck it in from the atmosphere. It should also have excellent resistance against ‘washout’ from water. Washout essentially displaces a lubricant, exposing the metal, allowing rust and friction.
Tribologically, a dominant issue with guns is that nearly all of the dynamics they operate under are sliding motions, with only a couple of parts rotating or spinning. These sliding forces and loads cause shear stresses, which threaten to rip through the bonds between lubricant and friction surfaces, allowing metal-to-metal contact. With the exception of your barrel, almost all the wear in your gun comes from failure of lubricant under shearing forces. Again, this has huge influence on lubricant design.
When examined under both slow-motion cameras and materials analysis, very few guns exhibit uniformly even travel of their slides or bolt assemblies along the friction surfaces during movement. These parts, to a greater or lesser extent, often 'bounce' along their course of travel. The consequences of the somewhat instant application of energy in firing a round, combined with this bouncing effect, are to bring relatively large amounts of energy and ‘load’ down onto small surface points.
This can cause what is known as ‘shock loading’. Unless a lubricant is engineered for shock loading, these forces can allow a moving part to slam right through a liquid film and allow metal-to-metal grind and galling. Shock loading of parts easily squeezes or splashes fluid-film lubricants right out of the metal asperities. In doing so, it can cause several problems, including excessive wear, as well as outright stoppages and malfunctions. It also often places excessive energies on surrounding parts that may not be engineered to withstand these forces, especially over extended periods of time and cycling.
Gun actions don't travel smoothly - they tend to violently bounce along the course of travel. This can slam parts right through fluid films, requiring lubricants to be designed to withstand both sheer forces and shock loading.
This is one key area where perception and reality often diverge with gun lubricants. It's not uncommon for a shooter to clean and lubricate a gun at their work bench, and marvel at how smooth the action feels. Unless done with great, instant violence, these workbench cycling forces do not accurately replicate what is occurring inside that weapon during fire, either with the ability of sliding motions to cause sheer force, or in replicating any shock loading that may occur. Especially with oils on sliding surfaces, it's difficult to protect against shock loading, and often nearly impossible without a proper additive package.
A properly selected additive package is critical and fundamental to any professionally engineered lubricant, which we dedicate a short segment to. However, before covering additives, it’s important to dig a little deeper into core concepts of lubrication.