I'll take a stab at answering my own question re the loading manuals' seating depth recommendations.
It may be that leaving a bit of the bullet exposed means that the crimp will be into the bullet rather than over the end, thus reducing the risk of the bullets in the unfired rounds seating too deeply into the case upon recoil. A reasonable safety concern for the publishers.
Note that the factory wadcutter ammo has a crimp at the base of the bullet, for just that reason, something a handloader can't duplicate.
Paul
If you want the crimp at the base, use wadcutter brass. The crimp doesn't go away after firing.
