To my understanding, the reason lead bullets shouldn't be shot in Glocks (or most H&K pistols for that matter), is because of the polygonal rifling. If you look down the bore of a Glock you'll notice an absense of traditional lands and grooves, and instead see that the bore, if cross-sectioned, would look like a hexagon (for .40cal and smaller) or an octagon (for the .45cal). The problem with the rifling is that lead is MUCH softer than copper jacketed bullets, and the polygonal rifling doesn't grip the lead as positively as standard rifling, so the bullet can essentially "skip" across the rifling briefly before the rifling grabs it enough to make it spin. This can cause a great deal of sheared off lead to build up right at the throat, which as most know is the worst possible place for lead (or anything else for that matter) to build up, essentially creating a partial bore obstruction. Eventually, if allowed to build up enough, or if jacketed bullets (which won't 'give' under pressure like lead will) are fired after the lead bullets, dangerous pressure spikes can occur. This combined with the partially unsupported section of the case due to the generously sloped feed ramp common to Glocks, makes a badly rupture case and blown up gun (the infamous "Ka'Boom") even more likely.

This is why Glock, and the common wisdom of the Glock shooting community, cautions against the use of lead bullets in Glock handguns with stock polygonally rifled barrels. If you do want to shoot lead in a Glock, it is wise to buy an aftermarket drop-in replacement barrel with conventional land-n-groove rifling.

Mike