Montanan Smith and Wesson, Colt, Ruger and a myraid of others have been posting that warning for over thirty years. It is CYA on their part. I have seen as a range master for 20 years THOUSANDS of reloaded lead bullets fired thru police service Glocks. I am a Glock armorer, certified BY GLOCK, and have fired a Glock 17 that had over 300,000 rounds thru the barrel and slide. SOME WERE LEAD. The tenfer finish on the barrel and slide is far harder than anything American gun manufacturers make, and it will NEVER wear out. It I believe is on a Rockwell hardness scale 1 point under a diamond. If the EPA ever allows it into the U.S. it will change gun manufacturing here forever.
That 300,000 thousand round Glock by the way was furnished by Glock at an armorer school, and had been at that time thru 3 receivers and numerous triggers. At that time the receivers were about $37.00 each. You could shoot it all you wanted, and Glock would furnish the ammo, and document the rounds found. It was truly amazing.