It is mostly hype and not restricted to Glock... all manufacturers will say the same thing about reloads. "Don't do it or we will not honor your warranty" I don't blame them because people are generally pretty stupid in that they often prefer "trial and error" learning rather than scientific reliance on existing bodies of study.

A glock, or any other hangun that "was" a 9mm but was redesigned for .40, has a partially unsupported chamber. Simply means that this is the weakest link in the chain if for some reason pressure is greater than the gun was designed for. Glock is not any more susectable to this condition than any other manufacturer...they simply produce 10 to 1 over any other .40s&w manufacturer

There are many warnings about .40 S&W because there were problems with them blowing up in the past. People loaded and unloaded the same bullets over and over without shooting them (police officers/security guards/HD guns)...this caused the bullet to eventually drive farther and farther in the cartridge. Because it is a high pressure round and it has a relatively small capacity the space restriction would cause pressure spikes that would go 80, 90, 100+ cup...causing a kaboom.

The solution? manufactures actually glue the bullets to the cartridge to help prevent this situation these days. It was too little to late before all sorts of hysteria about how crappy the cartridge, the gun, or the ammo could be with respect to anything .40s&w. That warning was a knee-jerk reaction because at the time no ammo manufacturer wanted to say what was causing the problem because they provided half the US police force with these guns. Self regulation and warnings was left to independant ammo makers like Buffalo Bore...which is known for pushing things right to the limis of SAMMI anyway unlike mainstream that stays 10-15% under.

If your brass bulges...it is loaded too hot or the gun is dangerously unsupported beyond the initial design of the gun. This is visually very obvious, but at the same time not common at all even with Glock handguns. Simply avoiding reloading .40 because it may have been shot through a glock is being way too conservitive. Visual inspection, sorting, and quality control is priority +1 with reloading. If you ever see a blulge there is something wrong with the gun and you need to put it out of commission until you fix it. Again...it has nothing to do with Glock (I am not a fan boy/kool ade drinker either...think they are utter utilitarian snit).

When brass is expanded and contracted it actually hardens...the amount that the brass bends with respect to a "short/unsupported" is minimal to the expansion that the mouth of the cartridge makes when the gun fires. Thus keeping the indication of warn out brass to neck splits...not Kabooms.