I'll seat my bullets close to desired COAL and turn my seating die down in increments and measure the COAL. I would turn the seating stem down, measure.. nothing, turn it down more.. nothing.. a little more.. and the bullet seats too deep?
This is happening because of stiction. The pre-seated bullet isn’t moving in the case neck until the displacement becomes far enough to allow enough force to make the bullet slip in the neck again. Until then, you’re just flexing the die, brass, and press, and not overcoming the neck tension.
If you’d seat another bullet, instead of reseating the same bullet, the position would have changed with the first adjustment.
Can I use a collet puller to pull the bullet back out and reseat it again.
I don’t like using a collet puller for this, as it has to squeeze the bullet, maybe influencing the jacket to do so. Inertial puller will cause less bullet influence.
Equally, there’s always consideration that a bullet which has been seated is different from a bullet which has not. It’ll seat slightly differently, and may even shoot differently.
Additionally -
@jmorris may be along soon to offer a photo of his dial indicator jig he made for non-micrometer seating stems, but the depth gauge on any vernier calipers can be used to do the same. When you find your land-kiss length, note the COAL of that bullet, pick your desired jump, then use the caliper depth spindle to measure the height of the seating stem, zero the caliper, then reduce the length down to your targeted jump.
Finally, if your stem doesn’t fit your bullets, fix it or replace it, or both. For example, if you are shooting VLD’s/ELD’s with sharp points and long radius count ogives, drill a recessed mortise into the center to clear the tips to make sure you are only making contact with the ogive. If you’re getting limited contact between the ogive and seating plug, even with a ELD/VLD stem, obvious by leaving a ring on the ogive (or worse, collapsing/deforming the nose), then either lap or bed the plug to better fit the bullet profile.
Two questions, however: 1) How much neck tension do you have? And 2) how are you cleaning your brass and 2.1) if you’re using any wet cleaning method, are you lubing your necks? Or 2.2) if you aren’t cleaning, are you brushing your necks?