RecoilRob,
It doesn't have that much to do with a tight bore. What it shows is that the case is expanding enough to release the bullet, rather than restricting it, thereby driving the pressures up. If the bullet is not restricted from starting to move, a slightly larger bullet will not unduly raise pressures. On the otherhand, if something restricts it, such as case neck/bullet diameter too large for chamber neck or too long case jammed into leade, pressure must rise enough to overcome the resistance before the bullet can move. This results in a higher peak pressure. If the bullet is able to move without undue restriction, before entering the barrel, the volume of burning gasses is increasing which moderates the increasing pressures. By the time the pressures peak, the bullet has been sized to what ever the barrel diameter will allow. This occurs by the time the bullet travels the length of it's shank into the barrel. This description of what happens may not be precise, but is how I understand it. This is not something I discovered, it comes( as I understand it)from experiments done while developing the German 8x57 IS cartridge from the 8x57I; before WW1.
Mike
