Right, and I believe that is Mr. Guffey's point when he says you don't move a shoulder back. The sizing die is actually an extrusion die, flowing the top end of the shoulder into the neck, and the top end of the case body into the shoulder. Thus, the shoulder brass doesn't actually relocate down to original position, it gets displaced.
CW308,
Mr. Guffey's point is that if you set a shell holder on your bench and then put a GO gauge in the shell holder and rest an FL sizing die on the protruding part of the GO gauge, there should be a 0.005" gap between the mouth of the die and the deck of the shell holder. This is because sizing dies have to be made short to allow for case spring-back in a case fired in a long chamber. I've resized cases fired in heavily worn club Garands, some undoubtedly near military Field Reject chamber length (which is longer than commercial Field Reject) and seen as much as 0.005" difference in the head-to-shoulder lengths of their cases after a single trip through the sizing die. So it makes sense for die makers to build that much extra squeeze into them.
It is also because conventional wisdom is that, especially for self-loading guns, you want the shoulder datum to wind up about -0.002" shorter than minimum chamber length to ensure reliable feeding in a minimum chamber. If you measure new commercial ammunition, is usually has 1.627-1.628" head-to-shoulder datum length, which is shorter than a minimum chamber headspace by -0.003" and -0.002", respectively. If I had one of the aforementioned long-chamber Garands, I would not try to get there. Just extruding the shoulder enough that the datum diameter measures -0.002" shorter than it was as-ejected by firing would be good enough.
Touching the lands
There is a persistent belief that touching the lands causes a sudden jump in pressure. This is not accurate. The late
Dr. Lloyd Brownell measured the effect and found pressure increased as you got near the throat, so being 0.002 off the lands does not make pressure appreciably lower than kissing the lands does. But by the time you are 0.030" off the lands with a tangent ogive bullet, you see about 20% difference in chamber pressure. (QuickLOAD's recommend start pressure allowance puts it more like 18% greater, so I figure that's pretty average.)
What Dr. Brownell did was use a round nose bullet to make his measurements. The round nose has a very gradual taper from the shoulder (forward end of the cylindrical bearing surface) to the hemispherical nose. That means the annular gap between the bullet and the outside of the freebore increases in area very gradually as you seat the bullet deeper. The fact he had to move this bullet much deeper to get a change in peak pressure than you do with a spitzer nose bullet, which increases the annular gap much more for each increment of greater seating depth, caused him to conclude it is the amount of gas bypassing the bullet into the bore after the case neck expands and before the bullet moves forward to obturate (seal or block off) the bore that was causing the change in pressure rise and its final peak value.
So, while the amount of bullet jump you need with a spitzer nose to reach the minimum in your gun's pressure vs. seating depth curve will be much less than the quarter inch the gradually tapered round nose required in that plot, and while your pressure difference will likely be a larger percentage than for the round nose, nonetheless, seating just a thousandth or two back won't get you much less pressure than jamming does because the not much gas will bypass the bullet before it moves that short distance forward.
