One of the beauties of written material is that we can read it several times to make sure we understand what was written. Very seldom can anyone get all of the meaning of a technical post of any length by only reading thru it once. I know I can't do it, there is just too much data to absorb in one pass.
Your scenario of spring cases but no boolit spring back is too cool, and would explain why some people have problems with the boolits getting sized by the FCD. Problem is it don't work like that.
We are talking about a Lee Factory Crimp Die with a Carbide sizing ring in the base. I have 3 of them and they all measure .4225. Measured with 3 sets of high dollar calipers from my shop. If they were .420 the case head would not go thru them and we wouldn't be debulging them with Lee FCD dies.
I don't know where you got the data that brass springs back any significant amount, however if that is true then none of our reloading dies would function correctly.
I am sorry but this information is false pure and simple. Sorry if this offends, but it is simply not like that, period! Not gonna go there.
There is 100 guys here who will back me up on this one!
I've got 30 years in the machine shop and another 35yrs reloading ammo. I would have noticed long ago if this were true.
If a brass case springs back any more than .0003 (3/10,000ths) I would be surprised. .003 springback is rediculous as there would be no interference with the bullet to hold it in place. The crimp can not be relied on to hold the boolit in place by itself. You need an interference fit between the boolit and the case. It is called "Bullet Pull."
My Lee Carbide Full Length Sizing Dies measure .4175 and .418 all of the cases I have that have been sized with the .418 die are .418. The .4175 one was bought as an experiment and never used. That's where I got the numbers in the post. .418 -.020 = .398 or .003 interference with the .401 boolit. This little press fit will not size a boolit unless it has a large amount of Velveeta cheese alloyed into it.
If the case sprang back .003 then you would have to size to .415 in order to get the .418 necessary to have sufficient bullet pull to hold the bullet in place. If the case was that hard it probably would size the boolit. If every die set had this much variation built into it, we would never get it right. My god what would happen if we annealed the case before it was sized? would it still spring back? Don't think so. Now our undersized dies are useless. WE would chase the size of the case until nobody wanted to reload ammo any more and the liberals would get their way and take our guns away, and nobody would care since they were such a PITA to load ammo for to go shoot!.
A .40 S&W case DOES NOT spring back .003 of an inch under any circumstances. If they did none of them would hold a bullet at all. It physically can not be so. Why would you need an expander die?
Brass is very maluable and it sizes nearly exactly to what it is shoved thru or what is shoved into it. That's why they make cartridge cases out of it.
I have post sized many brass tubing parts with the end of a Number Drill in a lathe. We use what ever size we want the part to turn out and with a little oil it comes out just like that.
The only possible circumstance I can see brass springing back after sizing is if the brass had been loaded a zillion times and had workhardened to the point of near failure. Even then I doubt you would see more than .0005 (1/2 thou). it would break up first, as it would be too brittle to squeeze down to go thru the die.
Now if you are talking steel cases, that is a completely different story. But not brass. Sorry no way.
Randy