I will offer....
A. Take the die, run it down to touch the shell holder at full extension.
B. Back it off two turns.
C. Select a lot of both mil and win cases.
D. Measure them at neck, shoulder, and base, as well as fired headspace. Record this
E. Size 5 cases mil, 5 cases Win, liberally lubricated.
F. Note the difference in relative use of force in sizing.
G. Repeat D after step E. Record...
H. Reset the die deeper by 1/3 turn.
I. With different cases, repeat EFG.
J. Reset die 1/3 turn deeper... repeat I...
K. Repeat J...
Now you will have 5 cases mil/win sized at 4 die settings, measured b4 and after sizing.
With the recorded results, you will know exactly the difference, different brass sizes at each setting.
And bc you are still a full turn above the shell holder, you should not have destroyed any brass....
And then... using 0 as shellholder touches die.... if you take the 2 turns above zero, 1-2/3 ^0, 1-1/3 ^0, brass and size each again, at 1 turn above 0, you may see different results because each will take a different level of force sizing. This seen difference will help show you the setting needed for each lot of different brass.
Now, a bulged shoulder as shown in your picture can be caused by more than one thing...
A. As stated, case mouth is hitting too soon bc case is too long. There are other ways to know this.
B. The size of mil case dimensions fired in a fat chamber, may move lubricant differently than slimmer cases... too much lube on a fat case neck and shoulder moves down under pressure, and in turn moves the shoulder in strange ways.
C. Bc the case is so fat, more force needed to compress the brass etc, the press flex may be great enough that the shoulder hasn't sized down yet because press flex gave added distance. Leaving the shoulder pressed fatter at that point in the process, where. 003" more sizing would resize the shoulder width to where it's not visibly fatter.
c-1. Another. 002" compression could then show the too much lube wrinkling of the shoulder...
There are several ways to find the culprit depicted in the picture.
If you have this problem (B), the A-J die setting method will show you excess lube movement to the shoulder and answer that as well as provide the other data.
Best to you,
vr