There is a bit of error in thinking here. The facts are as follows.
Given the load has enough pressure to obturate the bullet hardness being used, the bullet upsets and fills the cartridge tightly, while the cartridge is tightly pressed against the chamber walls, BEFORE THE BULLET EVEN STARTS MOVING OUT OF THE CASE.
Knowing this answers all the questions which have been asked here. Pressure doesn't change with bullet diameter when shooting cast. Because chamber pressure with any loads which might leave a pressure concern, the bullet is a tight fit before it starts moving. (Jacketed are different critters, and diameter does change pressure.)
Therefore, gas checks and bullet bases will be deformed the least if the bullet is fitted as closely as possible to the chamber and throat. If it is undersize before firing, both gas check and base will expand and go out of square while doing so, then be sized back down to groove diameter as it enters the rifling. When a close fit to start there is no out of balance slumping of the bullet, just the sizing as it comes down to barrel size. ---- So, tight chambered guns will be more accurate than those with sloppy chambers, but sloppy chambered guns will turn in their best accuracy with bullets fitted as outlined above.
About gas checks coming off inside the barrel. Major error in thinking here, and the first writer I know of who is probably the one who started the idea, was Elmer Keith, no less. Facts here are, that with a 44, the one Elmer was concerned about, there is 4 tons of pressure against the check at maximum chamber pressure, and this pressure never falls below a ton while the bullet is still inside the gun, with stout loads. Very light loads it might drop to as little as 400 pounds which in my imagination seems to be enough pressure to keep the check, not just moving with the bullet, but PROPELLING THE BULLET! Solid facts here me friends. Nip the myth in the bud from now on whenever you hear it. When explained this way to anyone with a basic understanding of mechanical things it will be clear as crystal, and no arguments. Want a little more scientfic explaination? Calculate the square inch area of the base of whatever bullet you are interested in, and multiply by whatever you think muzzle pressure might be. Very few loads in centerfire rifles drop below 10,000 psi, as I remember it, from writings some years ago.
By the way. Elmer stated that gas checks got knocked off as they left the cylinder and entered the barrel throat. --- Fact is, with heavy loads they rivit out and fill the huge sloppy throats in modern revolvers, then get sized back down to barrel size. The powder blast coming out of a .005 cylinder gap will slit your fingers if you ever get them up there while you drop the hammer. Even from a 22 LR round which has a max chamber pressure somewhere around only 10,000 psi.
If you ever make the mistake of learning this the hard way, you will never again doubt that gas checks are clamped on pretty tight, and doing ALL the work of pushing the bullet out the barrel!
