Ian, speaking for InRange TV, was challenged by Steyr on the previous mud test they performed on the venerable and fairly well regarded AUG. They claim it wasn’t properly conducted to take advantage of the rifles ability to overgas itself for adverse conditions. So, Ian and Karl oblige with a new test given that proviso.
Several rifles, especially those using short stroke pistons, incorporate an ‘overgas’ setting to power the action through sub-optimal conditions of dirt, debris, and dryness (lack of lubricant). Several also use the same ability to have an ‘undergas’ setting for use with most suppressors that increase internal back pressure. These gas adjustment features are a well thought out addition, taking advantage of the available mechanics, and allowing a greater range of operational conditions.
But no system is perfect. They have trade-offs and weak points.
The AUG, when originally tested, seized up due to the intrusion of the mud into operating areas like the charging handle channel. That channel has a rod directly attached to the bolt carrier, mirroring the gas piston rod, and it does not have a great method for debris to leave.
Rifles whose designs do not seal the whole action system and give a good way for debris and gunk to clear itself will be more vulnerable to an ingress induced failure. Bullpups, despite the fact I like them, suffer from this weakness more strongly because the rifle bodies just offer more places for crud to sit and bind the moving parts up.
Does this make them bad designs? No. It does make it a known point for observation in the designs, a known mandatory maintenance point.
These are not a “real world conditions” test. It’s a valuable data point and it gives information for consideration. But if someone were instead to run the rifle through a course of fire on a muddy range it would be a more “real world” condition. Because, real world, people tend to keep their guns clean(er) and out of the muck as much as possible.
So why perform mud tests if they aren’t “real world” data?
Similar to why you proof load test a rifle. It’s an extreme case safety information point. What will a system do under an overly extreme condition and is there an easy way (or a way at all) to expediently fix a failure of the system if the condition occurs.
Debris ingress will always start messing with the tolerances within a machine like a firearm, there is a reason firearms run best when cleaned and lubricated, and ingress should be cleaned out at the earliest moment. This is firearm maintenance 101, keep the action clean and properly lubricated to work. The mud test data point gives an indication of how tolerant those tolerances are when thrown for the most extreme of external dirty conditions. But it certainly doesn’t test what the gun will do if mud gets in the barrel (hint: boom), which is another real world possibility.
The test effectively highlights how well the operating system is sealed, not just immediately around the bolt carrier but the whole exposed working mass. It effectively highlights the points in which debris will build up and bind components, giving maintenance data. It alone does not make or break how ‘good’ a rifle is though. That is a far greater data pool and a long series of (sometimes personal preference) questions.