One suggestion of what the "additional factor involved in Glock's warning" might be is that Glock barrels have a fairly sharp transition between the chamber and the rifling, and this area (known as the "throat") is prone to lead buildup if bare lead bullets are used. This buildup may result in failures to fully return to battery, causing the gun to fire with the case not adequately supported by the chamber and the breech not fully locked, leading to a potentially dangerous case failure. However, since this sharp transition is found on most autopistols this speculation is of limited value. The sharp transition or "lip" at the front of the chamber is required to headspace the cartridge in most autopistols.