Bullet base-to-ogive length has little to do with maintaining consistent seating depth. In contrast, bullet
nose length can have a significant impact on consistent seating depth because the two contact points critical for maintaining consistent seating depth are located on the bullet ogive. The two critical contact points are where the seating die stem contacts the bullet out toward the meplat and "pushes" it during the ram downstroke, and the point on the bullet ogive just above the top of the bearing surface where the caliper tool insert seats when we measure:
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Bullet length variance in the nose region between the two critical contact points can have an undesirable effect on seating depth consistency. In order to reduce this effect, one can sort bullets using a tool such as Bob Green's Comparator (
https://greensrifles.com/new-product-page), which effectively sorts bullet by the distance between the two critical contact points. Alternatively, if the bullets used have a fairly consistent BTO dimension, then the majority of any OAL length variance will by definition reside in the nose region. Thus, one can sort bullets by OAL as sort of a poor man's Bob green Comparator, as long as the BTO dimension of the bullet is consistent.
Once any obvious potential issues with the seating die stem and/or press have been ruled out, and the bullets have been length-sorted to maximize consistency between the two critical contact points, there are still other things that can be checked or tested if the seating depth inconsistency still remains. For example, excessive neck tension (interference fit) can affect consistent seating depth, so that is probably worth checking. The use of some form of neck lubrication may also be of benefit. The bottom line is that with issues of this sort, it is often necessary to evaluate possible variables (i.e. "causes") one by one until the culprit can be found and eliminated.