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Gunmakers have been putting the warning about reloads in their manual for at least the bulk of the last half century, maybe longer.
It's boilerplate cya for them, because, it should be patently obvious, the maker has NO CONTROL over what YOU put in their gun.
Essentially, the warning is a sop to the ignorant, and the ignorance of our legal system. These days, if you make a product, you not only have to tell people how to use it, if you do NOT tell people how NOT TO USE IT, you can be sued.
The S&W manual's warning in the previous post is the first one I have seen that tells you WHY they say don't use reloads, but the warning against them is an old thing.
Simply put, its the legal disclaimer saying if YOU use ammo that blows up their gun, THEY are not at fault. Because they tell you in the manual not to do this.