“lower chamber pressures”........

It’s not combustion pressure, per se, that causes the problem, but a symmetrically-shaped accelerating plug of gas.  When this impacts an obstruction such as a bullet (or snow, or dirt), the collision forms a region of extremely high pressure.  If the powder is not symmetrically distributed/confined in the combustion area, there will normally not be a problem.  In Dell and Helbert’s experiments, they were not surprised that they could cause ringing with un-confined powder by aiming straight up.  This barrel-ringing phenomenon has been well-understood for ~ 150 years.

CHRIS

If you want to use kapok to keep the powder in the vicinity  of the primer, just one little tuft of it 1/8 inch or more above the powder will do the job and permit the surface of the powder under the influence of gravity or loading movement to not be parallel with the base of the bullet. No need to stuff.