I read Ed Harris religiously.
His original article on handloading the 7.62x39 got me into the game, and I've since discovered I have a 2 MOA Bulgarian SLR-95 thanks to the technique.
As for obturation, yes it happens at the highest pressures. However, the bullet bumps up to fill the bore, creating a very tight (and essential for accuracy) bullet/bore fit. The lead bullet still presses nicely against the gas port as it accelerates towards the muzzle, and the copper gas check scraper won't clean the lead left in there. Does the copper gas check also leave some of itself in the gas port? Yup, I'll take pictures of the lead/copper fouling on my gas piston sometime - but the copper gas check is considerably harder than the lead bullet it's crimped onto, and less of it departs for the gas system.
I've experienced lead fouling in both my AK and SKS rifles, as my own hardcore experimentation with cast bullets led me down that path. Be they linotype, wheelweight, Lyman #2 alloy, or gas-checked variations thereof, Alox, Rooster, Javelina, or whichever lube, my gas piston and gas cylinder in particular were seriously lead-plated. That lead came from somewhere, and I might be off, but I'd wager it came through the gas port in the barrel vs. somewhere else. Just me guessin', mind you, but I also got some lead on the gas port scraper tool found in the average AK buttstock trapdoor cleaning kit. Hmmm...
I'd attribute Ed's aforementioned article to him having an AK with a nicely-chamfered, no-burr gas port. In a perfect world, that'd be hunky-dory. Even then, he still got gas piston and gas cylinder lead fouling, and he recommended using Marvel Mystery Oil to keep them lubed after lead removal. Anybody who shoots an AK on a regular basis knows that'll burn into smoke and carbon after just a few rounds. I note that cast bullet users also keep their bullet velocities down, which is fine if that's what you're looking for. Me, I keep my AK velocities around 2300-2400fps, which means the sight range settings are calibrated for the round. A heavier, slower lead bullet, even if it doesn't leave lead in the gas system, means you learn a new range table. Not a problem, just a learning curve. Ed's a neat guy, and I applaud him for trying the unusual, but in all my years at Camp Perry, I didn't see that many cast-bullet Garand, M14/M1A, and AR-15/M16 shooters. Maybe I just wasn't looking hard enough.
Again, I've farted around with all sorts of rifle cast
boolits beginning about 15 years ago. Bolt guns and recoil-operated autoloaders, no problems. Leverguns, Ruger #1, and Sharps, no problems. Gas guns, not worth the hassle.
Which reminds me, I've got a batch of Lyman 457132s I've got to cast soon, now that I have a new batch of beeswax for the lubrisizer...