Despite TightGroup's claim, how can the position of the powder in the case (particularly in situations where there's a fair amount of free space in the case) NOT affect the burn characteristics and therefore the pressure.
Here's some information from VihtaVuori's reloading guide:
2) Free space in the case
When using charges which leave large amount of free
space in the case, the shooting characteristics may vary
largely depending on where the powder is located in the
case. If the powder lies totally in the bottom of the case
(i.e. in the end where primer is), the muzzle velocity and
especially the maximum pressure become much higher.
The maximum pressure may even be doubled when
same powder charge is moved from the bullet end to the
primer end of the case. This can simply be demonstrated
by shaking the revolver barrel upwards or barrel downwards
just before turning it smoothly in horizontal
position, aiming and shooting. Also the recoil may
transfer the powder in either end of the case. This is
sometimes seen as a velocity change between the first
shot and the following shots.
The shot to shot deviations in velocity and pressure
are normally increased when using load which leaves
the cases half empty. For this reason such loads are not
recommended for target loads. The data below is tested
in a way that the powder is as much as possible in the
primer side before firing, and therefore, the pressures
and the velocities represent the maximum values which
were obtained using our test equipment and cartridge
components indicated in the table.
3) Risk for underload detonation
This risk is always present when using highly reduced
loads of any smokeless powder. The large free space in
the case may generate a pressure wave which can cause,
in the worst case, powder to burn as a shock wave, i.e.
to detonate, instead of normal fast burning process. The
extremely sharp pressure peeks involved in detonation
can destroy the weapon and may lead to serious injury.
All these loads given here are extensively pressure
tested and no sings of underload detonation were found.
We strongly recommend everyone to follow strictly
these tables to minimize the risk for underload detonation.