The question comes up because of information (or misinformation?) about using cast lead bullets in barrels with polygonal rifling, and this stuff has been floating around on the web and in shooting circles for decades.
It also doesn't just affect Glocks, there are several manufacturers who use polygonal rifling, including H&K, CZ, Bersa, and others, but we rarely hear anything about them.
There are two distinct issues that I've heard regarding this topic: Excessive leading of the bore and poor accuracy. The issue of leading has been shown to generally be a myth. No matter what kind of rifling you use, if your bullet and load does not match your bore diameter well, you will get leading. If you use a fast, hot burning powder, or push the bullets very fast, you can get leading. It's been demonstrated by hundreds of competitive Glock shooters who run (properly loaded) hard-cast lead bullets that leading can be reduced or eliminated, so that just leaves the second issue: Accuracy.
Unlike the misinformation about leading, there IS some merit to the issue of accuracy in polygonal rifling. Traditionally cut rifling has very sharp corners on the lands which really bite into the surface of the bullet and impart spin. Polygonal rifling features more obtuse angles in comparison (think gentle rolling hills & valleys), and while this works great on jacketed or plated bullets, the relatively soft hard-cast lead bullets do not 'bite' as well on these.
Because of this, it becomes much more critical that your choice of bullet fits the bore diameter perfectly, so that the body of the projectile fills the grooves of the rifling and spins effectively (This is one of the reasons why we offer multiple sizing diameters on many of our bullets).
With our polymer coated bullets, suddenly this isn't as big of an issue, since the surface of the bullet becomes much tougher, more like a plated or jacketed bullet. These bullets will engage the rifling effectively, even if they are slightly undersized. However, for the best accuracy, measure your bore and pick the bullet which fits perfectly or is .001" over the size of the max bore diameter. Alternatively, if you don't want to slug your bore and measure, we offer sample packs in multiple diameters so you can test fire them and pick whichever one groups best!