I have loaded and shot about 2000 PENN bullet hardcast lead bullets through my G26 Glock with no Ill effects, If you read the entire Glock manual cover to cover it does not even mention lead bullets. At least it doesnt in my G34 or G26 manuals. What it does say Is DO NOT SHOOT RELOADS. Thats because Glock has no way of knowing if your reloads are within saami spec for pressure. Its up to you to make safe reloads. Hardcast lead bullets work fine in a 9mm glock. I would not however shoot them in a .40 S&W glock the .40 is closer to the edge than the 9mm and many full power .40 lead loads without cleaning could drive up pressures enough to cause a Kboom IMHO.
Now if you own a Beretta 92 the manual does specifically say DO NOT SHOOT LEAD bullets. The rifling in a Beretta 92 is not polygonal its land and groove!!
And when I shot my Penn Bullet reloads in my 92FS compact, The rifling would have lead strips hanging from it after 25 rounds
At that point the Bullets would be hitting the target sideways
at 50 feet.
SCARRY it was. I dont know of any other handgun that does this and it has conventional rifling!!!! I think its because the Beretta has a very fast twist rate, too fast for lead.
I dont own the Beretta any more, sold it last year.
Now I also stopped shooting lead in my Glocks, because it does require some scrubbing to get the lead residue out of a Glock barrel. Instead I now shoot 125 grain Berry's plated Hollow points over 5 grains of Unique, a safe accurate load that runs 1050 +- 23 fps out of my G26 and a little faster out of my G34. The Berry's do leave a little copper residue.