The problem with leading and Glock bores isn't that it's certain to happen, but rather that it's very difficult to predict and so there isn't any good rule of thumb about what you can and can't get away with.
One book (The Glock in Competition) records the results of testing using pressure measuring equipment that showed that "identical" pistols shooting identical ammunition leaded dramatically different amounts resulting in dramatically different discharges. One went several hundred rounds with very little pressure increase while the other pistol which was apparently identical reached dangerous discharge pressure levels after only a couple of boxes.
People shoot lead in Glock barrels and most of them get away with it, especially if they're careful and know what they're doing. Some don't get away with it, and not even being careful is always enough to insure that you'll avoid an incident.
The first person who publicized this issue was a forensic engineer. He blew up his Glock using lead bullets. He had put around 23K lead reloads down his bore without incident before he finally shot a few too many rounds through his pistol in a single practice session and crossed the threshold of what the pistol could handle. Based on what I've read of his work, he was a very careful person but it didn't keep him from ruining a pistol. He later analyzed his pistol to see what caused the explosion and determined that the bore was sufficiently leaded to cause discharge pressures to skyrocket. He was able to duplicate the effect later in a test barrel.
But where is there a place for lead to build up in a Glock barrel?
That's actually a big part of the problem. There's not any good place for it to build up. If you get any appreciable amount of leading it immediately starts competing with the projectile for bore space. Something's got to give when that happens.
The forensic engineer declined to give any advice on how to shoot lead bullets. He switched to plated bullets after he concluded his testing.
What follows isn't advice on how to get away with using lead in a Glock bore, nor should it be considered even tacit approval of the practice. It's merely an admission that some folks are going to do what they want to do no matter what.
I don't presume to step in where Passamaneck feared to tread, but I think there are some rules that can be followed to REDUCE
, not eliminate
the chances of an incident.
Don't push the length of your practice sessions with lead bullets and frequently check for leading, especially anytime you try a new pistol or a new load.
Don't use maximum loads.
Don't EVER put ANY jacketed rounds through a Glock barrel that's had lead bullets shot through it until the bore has been thoroughly cleaned and all leading removed from it. I read a posting on another forum many years ago by a person who blew up his Glock 19 with 51 rounds. 50 lead rounds followed by a single jacketed factory round. The gun came apart spectacularly when the jacketed round tried to fit down the constricted bore. This practice can cause damage even in standard rifling. Speer's ballistic's expert, Allan Jones, recommends against the practice in all firearms citing his examination of several firearms damaged by the practice and as I mentioned above, Beretta includes a warning against it in several of their pistol manuals.
Don't assume that just because you've never had a problem before you're home free. Little things can make a big difference. If you change loads/load components or start using a different Glock pistol or replace a barrel, don't assume that everything will still be the same. As mentioned above "identical" Glock pistols can behave very differently in terms of leading.
Finally, be aware that given the unusual nature of Glock rifling, leading may not be easy to detect. One shooter on another forum bought a Glock 17 for a pittance (under $200) because the barrel was "shot out". I suggested that it wasn't shot out that it was merely badly lead-fouled. He checked again and was certain that it wasn't lead-fouled. It wasn't until I convinced him to carefully test the inside of the bore by trying to gently scratch it with a small screwdriver that he verified that the smooth interior of the bore was actually the result of lead fouling. This story, by the way, is a testament to the sturdiness of the G17 and how overbuilt the original 9mm Glocks were. I do not believe that a .40 S&W Glock would have stood up to being shot with that much lead fouling in the bore.
...they for sure will not cover them if you use non Glock parts...
I haven't ever heard anything like this from Glock. I can't imagine why the use of a drop-in part would void the Glock warranty, but if you have information to the contrary from Glock, please post it.
...as the less tight Glock barrel will let gas around the bullet...
While the chambers may be a bit on the large side, the bores are actually tighter than found in most standard rifling. As mentioned above, that's part of why they don't tolerate leading well. The tighter seal between the bore and the bullet is also part of the reason that polygonal rifling is generally accepted to provide slightly higher velocities.
You are correct that an undersized bullet will tend to lead badly due to gas blowing between the bullet and the bore.