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Beex215,
It is REALLY important to specify what cartridge you are loading when you ask a question. Even your answer that you load 9mm and .308 Winchester really doesn't help us give you the correct information, because those two roounds are VERY different with respect to over-all-length / seating depth relationship to pressure.
As others have stated, 9mm is very sensitive to seating depth. A quote from Speer manual # 10 says "... loads that produced 28,000 CUP went to 62,000 CUP when bullets were purposely seated 0.030" deeper!" (page 349). The effect is due to the very small powder space remaining under the bullet in this small case (which causes a small change in seating depth to make a big change in the ratio of powder space for the two seatng depths), coupled with the very non-linear burn rate of smokeless powders as a function of pressure while burning.
Bottleneck rifle cartridges like the .308 have a much larger powder space, so the ratio of powder spaces remaining for seating depth changes like 0.030" are not really significant to pressure. What is significant in the .308 is how far the bullet moves before it encounters the rifling and is inhibited in its acceleration to some extent. When the bullet is not accelerating as rapidly as before, it allows the pressure to build more rapidly, which causes the powder to burn more rapidly, which builds pressure more rapidly, etc. until the pressure is high enough to accelerate the bullet fast enough to stop the pressure buildup. It is a very dynamic process that is hard to model and dangerous to assume follows simple non-dynamic relationships like the "Perfect Gas Law." Unclenick's graph is a good EXAMPLE of the effect this has on pressure, but different guns, powders, bullets, etc will give somewhat different relationships of pressure with seating depths for the same cartridge. And different cartridges will have even more variation in the relationship.
SL1
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