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and what gun parts do they lubricate?
There are lots of moving parts in guns. Any place two parts move together, they should be lubed. Even non-friction areas need corrosion protection, so those areas should get a very thin coating of oil as well. Remember that the oil itself isn't doing much to protect the metal, it is the corrosion inhibitor additives doing most of the work.
Many people compare guns to engines or other parts of a car. These are false comparisons. Other than at startup/shutdown, engines and other parts of cars mostly operate in the hydrodynamic regime, due to their spinning parts.
Other than Gatling types, guns don't have spinning parts and operate wholly in the boundary regime. This is why EP and AW are important with guns, especially with stainless steel. Surface reacting EP additives tend to smooth the asperties on the surface of the steel, lowering friction, and improving reliability.