Larry,
How fast it happens is immaterial. And you missed my point. Obturation takes place at the base of a bullet cause that's where the pressure is. But with a tight fitting check and one that is sized down, no obturation ever takes takes place. It simply begins resisting obturation. Big difference to balance. And is this the key to powder speeds?
What is important from that is that you can't get to stage two without having gone through stage one first. That's all. And if you are going to deplete lube from any area on a bullet, the base area shouldn't be it. Especially if you plan on pushing ahead. This was just to get folks thinking about why accuracy goes south in a gradual fashion once the sweet spot is passed. That .... and to aggravate you. It's another theory. But it is one that Veral shared with me as he said he was surprised my 154 grainer was able to reach that velocity level since it was made for low velocity work. (narrow check groove) And he is right, it's sensitive to bore fouling with a slight temperature change.
What really holds a check on a shank is the rifling indented into the lead. The taller the rifling and the tighter the base, the better the grip. Surely you aren't naive enough to believe that a little teeny little lip inside of the mouth of a check that is in line with bullet rotation is going to hold all forces and brave 2000 MPH winds making all the difference? That's why you don't see much difference in brands. But that lip could be strong enough to prevent that check from pushing forward at stage one until it does get tight since it happens so fast.
As I go up in velocity, I believe fouling fills this space and tries to remove the check. Possibly while still in the bore. And this will throw the base outta square causing tipping. And this fouling is not uniform so it causes unbalance until it is flung off. And it would so happen that the faster you go to create more fouling, that the more you would collect. Which is exactly why lube or the type of lube makes a difference to accuracy.
That's why I like tight fitting checks and reasonable groove width. Too wide and you weaken the base area.
In Jeff's first picture, the helicopter check on the one bullet made me wonder how the check shape and condition would affect drag. I never thought about that in our discussions. We generally only think of the front of the bullet for ballistic coefficient while the GC is a common variable. Jeff was doing alright down low with the aluminum to the range he was using. But did a lot worse at 2400 fps than Larry's 3.77" group at the same velocity and RPM level. So something else wasn't in .... dare I say it .... check.
That's why I am thinking.