Due to the style of rifling in the factory Glock barrel, it is said that it fouls easier and can cause high pressures when this occurs. I have been shooting the same Glock 17 since 1989, my dad now shoots it as I have moved to G34, but I have shot eveythign in it including lead, lots of it, as have I with my G34. There are several stories as to why this is. Recently, I spoke to a retired police armorer who is very familiar with the Glock and he told he that this is being told due to liability reasons, so who really knows. Most will tell you to shoot jacketed/plated, I prefer it myself, but I am not afraid to shoot lead in my Glock, I have not had a problem.