Safety isn't the issue, it's more "functional" I guess is a good way to put it, it's more about building up excess crud in the chamber using .40 S&W. If I were to go shooting with both kinds in my bag, I'd shoot 10mm first, then maybe go with .40, but not the other way around. I would never do it in say a 1911, but I do think it's fine in a Glock. But yeah, it's better all around to just get a conversion barrel, I think.