SO. Contrary to popular belief, Old Dogs can learn new stuff. Rather than continue my belief about the possibility of cast lead bullets expanding upon firing, I went researching.
It is true that cast lead bullets can expand upon firing. CAVIAT: This phenomenon requires the lead alloy be carefully for Brinnell Hardness to the pressure level of the cartridge. If one is shooting common commercial cast lead bullets the probability of those bullets obdurating at the pressure levels we shoot at is ....... Nil. If one is shooting Bevel Base bullets the probability of obturation is ..... zip. Likewise, shooting gas check bullets .... Nada. Expansion after passing thru cylinder throats ..... only with dead soft bullets.
Shooting undersize bullets and expecting the bullets to "bump up" is a fools errand. Also, at no point in research could I find any reference to indicate unlimited obturation. The ONLY effective method for best performance is to shoot bullets sized correctly to the bore (Groove Diameter).
Those knowledge laden folks that toss the "fact" that lead bullets will "Bump Up" are only partly correct and only for an absurdly small number of CAS shooters.
Your text in red has already been addressed in a previous post. I researched the formula I mentioned and it is 1422 x Bhn of the bullet = the pressure required to begin obturation. That explains why the nearly pure lead bullets of ~6-7 Bhn used in cartridges 140+ years ago worked in (slightly) oversize barrels.
I don't why a bevel base bullet wouldn't obturate, maybe not a much as a flat base bullet due to the smaller surface area of the bullet, but the forces of physics would still be at work.
Sorry friend, like it or not, cast bullets can and do bump up, under the prescribed conditions. I don't recall anyone saying anything about "unlimited obturation". You can place a lead ball on an anvil and beat it with a hammer and it will only "obturate" so far. I'm not sure when/where/ and why that came into the discussion.
Of course the best course of action is to size the bullet properly. I personally size for throat diameter, or even a thousandth or two over. Sizing to groove diameter works well
only if and when the cylinder throats diameters are close to the diameter of the barrels grooves. This has been demonstrated repeatedly with Ruger SA's whose cylinder throats are very typically of less diameter than the grooves of the barrel. This makes for horrid accuracy and is why 'smith's offer the services of opening cylinder throats. I had a New Vaquero in 45 Colt with this very issue. At the other end of the spectrum are Colt SA's in 45 Colt flavor. Their throats are likewise notorious for being grossly oversized. The last 3rd Generation 45 Colt I owned had cylinder throats as large as .457". THAT is the very situation in which one must use a soft bullet that will bump up to fill the throats.
You are exactly correct regarding CAS shooters who need a projectile capable of hitting a large target at short range. Few concern themselves with accuracy, but you must remember that there are those of us at the opposite end of the spectrum who enjoy shooting revolvers at longer ranges, and to do so effectively the bullets must behave properly.
If you care to research the subject further, Dave Scoville, Brian Pearce, Mike Ventrino and the Cast Boolit website are great sources of information and learning.
CHT