Tejano,
The pressure results I've seen indicate pressures start rising when rifle bullets are seated pretty deeply, close to 1/2" deeper than out to the lands, or maximum magazine length. Probably that's when powder compression starts to play a role, but also probably varies with the type of powder, primer, etc. I can't even remember ever seating rifle bullets as much as 1/4" shorter than normal, so I've never found any need to reduce the powder charge.
Quite often velocities drop measureably as bullets are seated farther off the lands, though not always, especially when bullets are seated only a little deeper. This also depends on the particular bullet, powder, primer etc.
Crimping may have some effect, but I've never seen enough difference in velocity to assume it does. But I don't crimp rifle bullets very often, even in some cartridges where many handloaders assume it's necessary. Consistent neck tension on the bullet normally has more effect on both accuracy and pressure than crimping.
