Reloading pressure and velocity

This may be opening pandora's box but what is the relationship between powder burn rates and the pressures developed.

It is my understanding that higher pressures lead to higher velocities.

Furthermore accuracy should be a function of the standard error associated with velocity/pressure but it should also have some impact from the ballistic coefficient(BC) of the bullet used. I have noticed that published BCs drop dramatically for many bullet designs when velocities over 2800 fps are used.

When I made up the loads for my 270 accuracy went out the window when I raised the powder charge above the one that produced 2900fps on Zeisloft's chronograph. I did not see any visible signs of incomplete powder burn nor did I see problems with overly high pressures. I wish I had a chrono to tell if the velocities on these loads were consistent. I am left with two hypothesis:

1. Higher powder charge not consistently burning completely. This would be verified by erratic muzzle velocities resulting in poor accuracy.

2. Powder is burning fine but velocity is too high for selected bullet. Muzzle velocities would be consistent but accuracy remains poor.

Solution:

1. Change powder to a faster burner?

2. Change bullet?