I'm sorry the realities of the world are inconvenient, but just because we all really wish it was easier to derive meaningful results from limited data, it simply is not the case.
If the item of interest has been manufactured, it is not some newly observed phenomena like we have just observed for the first time the mating ritual of the humpbacked whale and the male performed a tail splash after mating so therefore we jump to the conclusion that all male humpbacks do the same.
In the case of the H&K (and S&W, SIG, etc) this is an item that is manufactured using a formalized process with quality controls, inspections, and tests for functionality before the product is shipped. The odds of getting an example that is outside of the companies tolerances are very low if they have a reasonable quality control process. One might argue that it has not been shown that the firearms that have failed have good quality control, but if we go down that path it is more damaging to the perception of the company than the failure to run in the gauntlet.
In every case the firearm was tested for function. In each case the firearm worked. After the firearms failed often they were cleaned and tested for function and the firearm resumed normal operation, and then when ran through the gauntlet the item failed again. If the firearm was broken during the test then cleaning it would not have put it back into operation.
The tests are outside the tests performed by the manufacturers unless the manufacturer has developed the product for such conditions. Firearms that are developed for military contract probably have been at least water tested and drop tested.
Therefore the test is like this, "We don't know if the firearm was designed to work in these conditions or not, but lets see what happens."
The reasons for the failures would be interesting. I have seen video of a semi auto pistol firing and a spent casing fly up in the air, come down and land in the open chamber because a following round had just been fired and the spent casing land right in there. It could be that during firing a piece of sand was lying on the slide and during recoil it went up and down the open chamber and landed on top of the next round such that the extractor picked up the sand, etc. Right, anything could be the reason for the failure. So, what do you do, you run it through the gauntlet again. Did two rare events occur? Do it again. Did three occur? Suddenly what may have seemed rare is starting to show that it is not.
If it is argued that the firearm isn't broken in properly, that has inherit issues as well. If something is functioning properly and you use it, you have changed it. Things typically don't break when not being used. If you are of the paranoid type then consider this, the last time you took your gun to the range and you fired it and think, "I have proven this firearm is ready for use", but in reality that last shot was the use that caused the trigger spring to break and you don't know it.
I had a firearm that would jam all the time. From day one, first magazine. The manufacture told me to break it in. Here is what I was told. The recoil spring needs 200 rounds to properly break it in. Also, the recoil spring has to be replaced every 500 rounds according to our recommendation. I am like, "WHAT?". So, I have to shoot 200 rounds and then I have a window of 300 rounds before I have to replace the spring and do another 200 rounds. You know what that is? A cop-out by the manufacturer not taking responsibility because if they did there would have been a recall. The lawyers and bean-counters ran the statistics and decided that one dissatisfied customer would not be a significant cost to their sales.
The tests have meaning. Conclusions are not made, meaning that if the firearm was studied and was shown that sand enters through here, lands here, and binds up this, and causes that, and we can reproduce the result every time, that would be a conclusion.
If S&W says, "It wasn't designed to run after being submerged into water! Why are you people now requiring it? It will work just fine in your holster, on your night stand, in your glove box, stop being ridiculous!" Well, sorry, customers are fickle things. They have now seen something that performed better in a situation they will never be in, yet they like that it performed better, and if the S&W costs $450 and the Glock costs $475, what is $25 more to get the one that loves water and sand?
I bet MAC gets pushback from the manufacturers, maybe even to the point they try to scare him off from doing these things. I can imagine someone now saying, "Doesn't he own a gun store, make sure none of our distributors sell him anything. We'll see how well his store does with empty shelves. He is being stupid coming up with unrealistic situations and people are now acting as if that is the norm."
Well, who is stupid? The customer? I thought the customer was always right. ;-)