This discussion lead me to try a little experiment. It is much more difficult to express oneself in written form then it is to actually talk to someone and use physical examples (or hand and arm movements!), but here goes.
I pick up all of the range brass I can find. And, since the 270 Winchester is a popular cartridge, I have plenty of this once-fired brass in the nice boxes it came in. So, I took 3 pieces of this brass and tried each one in the chamber of one of my Model 70 Winchesters. I did not force the bolt closed. None would chamber although with excessive force, I probably could have got the bolt to close.
I use RCBS dies for this cartridge. I removed the neck expander so as to not stretch the brass during the expanding process. I sized one case with the die set “just touching” the shell holder (a factory shell holder--unchanged dimensions). This sized case would not chamber without some force on the bolt. I then set the sizing die to slightly “cam over” during the sizing operation. This sized case would not chamber easily, yet I could get it to chamber with a bit of force. The shoulder had bright marks on it, as did the first case, where it rubbed the chamber walls when the bolt was slightly turned. I then sized the next case using my shell holder that has had a thousandth or so removed from the top. I screwed the sizing die down to just touch this shell holder. This process will push the shoulder back ever so slightly. I then put it in the chamber and it chambered with little or no effort. There were no bright marks on the shoulder.
I think what happens in some cases, as I described above, is the fact that the chamber might be the correct headspace (length), but the width of the chamber might be excessive, although still within specs. If one thinks about a solid piece of brass that is 3” long and 1” in diameter, maybe this concept can be understood a bit better. If this piece of brass is size to 7/8” diameter, the length must increase—that same amount of brass must go somewhere. Picture the cartridge doing the same thing. Yep, it might fall from the chamber easily after if is fired, BUT, when it is sized, the width would be lessened so the length would get longer. Or you have a case that is too long for a short chamber. That is where trimming a bit off of the shell holder allows the case to enter the sizing die a bit farther, and thus push the shoulder back—no more bright shiny marks on it when the bolt is closed and the round chambered. The difference between the case and the solid piece of brass is the fact that the case can move to a new shape by decreasing the internal volume of the case. Maybe this will help clarify my thoughts on cases not wanting to chamber properly after sizing…BCB