Lets take for instance the 300 grainer in the .338 win. mag being seated way down in the case and buried into the powder. Is alot of unburned powder around the sides/shank of the bullet blown by the bullet before it seals in the rifling verses a bullet with the base seated even with the bottom of the case neck and no powder around the shank? Does it make any difference? Does the base of the long bullet being deep in the case cause better combustion of the powder because it shortens up part of the powder column? Or, would a very long bullet get more velocity if it's base could be seated even with the bottom of the case neck? Just curious about it.

I think the greatest effects on such circumstances are simply the effects of powder capacity. Secondly would be a slight increase in pressure due to a smaller case volume. Beyond that anything else is splitting hairs IMHO.

Yes, I've also been told that seating a heavy for caliber bullet close to the rifling also increases pressure so what is best?