"If full length sizing means adjusting the die down to the shell holder with an additional turn?"
But, it isn't. That's the "conventional wisdom" way to size any kind of cases so they will chamber and go BOOM, but it's not automatically the best way to "FL" size anything. Best way is to actually set the case shoulder back just enough to insure easy chambering. Going past that point unnecessarily increases case stretch and leads to the oft repeated "toss a case that has been fired fired times", etc. IF the case NEEDS to be jammed hard against the die, fine, do it, but, if not, just do what's needed and smile.
"the shell holder and die must make 100% contact,"
Sounds okay doesn't it? But why? Fact is, there is no reason to think any slight die/shell holder contact offset due to less than precise die mouth grinding will automatically result in the case taking on that offset. Why should it? The major case siing resistance, or 'back push' on a case is from the tapered walls and shoulder, NOT the last thousant on the thick web and head. Thus, any rational small amount of "out of square" opening to the die mouth will have little or no impact on the resized cases. Some cases do have heads that are out of square from the manufactor and it's been clearly proven that no FL sizing technique can correct it, nor does firing at very high pressues rarely correct it. Stands to reason that no simple FL sizing technique can induce it either!
"Screwing the sizing die down further clearly isn't the solution because, as you say, the fired case fits already so what would be accomplished by making it smaller?"
Not so, it doesn't fit, that's why it's difficult to close the bolt. Anytime something won't fit into a space, it is by defination too large. ?? The ONLY way to make that case fit is to make it smaller. Since it's very unlikely that a (nearly) fully resized case is too large in diameter all that remains is that the shoulder is too far forward. Screw the die down, likely another sixteenth turn (4.5 thousanth) will do it.
