Mosin-Marauder,
Allow me to correct some of the above. Many bullets shoot best seated into the lands. Benchrest shooters did this routinely for years. When I went to Mid Tompkins Long Range Firing School, he explained that he and his whole family of world champion long range shooters use "soft seating", where in the neck tension is minimal and the bullet is seated out too far on purpose and finishes seating using the throat as the seating dies when they chamber the round. Berger recommended seating its VLD bullets 0.010" jammed into the lands for years.
The reason people fear seating into the lands, other than the problem of pulling bullets that T'Ohier mentioned (and that is a problem Tompkins mentioned; you have to be at a range that allows you to point the muzzle up while unloading if you are going to use this method), is that it raises pressure
if you don't adjust the powder charge to compensate. But if you develop the load from the start with the bullet in the lands, then it will work fine.
So, if you don't adjust the load, how much difference in pressure does it make? It depends on the bullet and throat geometry. RSI has a Pressure Trace instrument example of a 6mm bullet increasing pressure by 20% moving the last 0.030" into the lands. Dr. Lloyd Brownell's study of absolute pressure conducted at the University of Michigan in the mid 1960's has an example of a round nose bullet with its much more gradual taper only making around 10% difference at any distance off the lands and until seating depth became so great it started to raise pressure by taking up powder space. VLD's also have a fairly gradual effect on pressure because of their large ogive radii. This all happens because, according to Dr. Brownell, what causes the pressure rise is not friction, but the amount of gas that is no longer allowed to bypass the bullet before it jumps from the neck and seals the bore.
I suggest reading through
Berger's Tech Talk piece on making VLD's shoot well. They have to move them in fairly larger increments to see a difference. A bullet with a faster taper will probably want smaller increments of adjustment. The 20% rise measured by RSI is about as much as I've seen. A fellow using the same pressure instrument on another board saw no detectable peak pressure difference from touching the lands with a VLD and backing it off 0.030". This also is affected by how much robbing the case of powder space raises pressure as the bullet seats deeper. That and the bypass gas cutoff are effects that work in the opposite direction as the bullet seats deeper, so it follows that with some long enough bullets in some cartridges they could completely neutralize one another.
I also suggest that if you are going to start with a bullet in the lands, the powder charge be reduced a full 10%. That will reduce pressure between 20% and 30%, depending on the powder and bullet and chamber. That will keep you safe. (You should be safe in any case, as all this pressure difference is still below proof load levels; there just isn't any pointing putting extra wear and tear on the gun.) Work the load up from there. If you have a chronograph and you know what velocities you are getting from normal loads at normal seating depths, you should be able to get to the same velocity with the same exact powder, primer and case combination when you are touching the lands before the pressure gets too high.
