[Unclenick]

For a time in the late 19th century, billiard balls were made from solidified nitrocellulose (liquified by ether and built up in layers by dipping, I suspect), because the elastic bounce was good when the balls collided. I read they stopped using the material because these balls would occasionally have small surface explosions on contact like a cap going, but that it caused alarm where gambling was taking place. But I also heard the material just tended to age badly and crack. So choose your explanation. In any case, the stuff has hard elasticity that bounces well on contact with anything else hard.

My surmise has been that the drop tube simply lets the powder grains gain velocity so they have more propensity to bounce on impact with the case or the building powder column in it. Thus, they fly around in the case short distances, perhaps bouncing off each other a number of times before settling. That's the same thing vibration does to them, and vibrating granulations tends to settle them in more tightly packed orientations. That happens because gaps between grains have to be formed by bridging the same way a house of cards has each card bridge a span. A collection of such bridges is precarious compared to a stacked deck. The spaces give the elements room to fall into more tightly packed positions if bounce or vibration dislodges them, whereas a tightly packed element has nowhere to fall if loosened. Therefore, bounce and vibration are much more successful at eliminating bridging than at eliminating tight packing, and a bouncing or vibrating mass of elements tend toward tightly packed orientation (provided the bounce or vibration isn't great enough to toss the whole mass into the air, starting the packing process over).

Another way to look at it is a collection of grain bridges, forming a taller structure having more room to fall, have higher potential energy than tightly packed grains do, and randomly excited systems tend toward their lowest potential energy. It's the fourth law of thermodynamics in action.

Setting charged cases on the lid of your vibratory tumbler will also pack them down, though it takes extra time and you risk a few grains at the top bouncing out if the case is very full. But try it out. Put some charged cases in a loading tray and hold the tray against the tumbler lid and watch what happens. You can start your bullets in at the case mouths before packing to prevent bounce-out.

One fellow in the Precision Shooting Reloading Guide, (IIRC) found that a load that worked fine when assembled at home was giving him high pressure signs if loaded at the bench at the range. He finally figured out the vibration of transporting the loads was settling the powder enough to reduce its starting burn rate. That happens because the flame front has a harder time moving through tight spaces, so it propagates more slowly. This is why compressed loads don't always add as much velocity as expected.

By the way, spherical propellants don't pack nearly as much as stick powders do. I found WC852 (bulk grade H380) dropped through a 3 ft tube does not pack any more tightly than 4064 does dropped through a 4 inch tube. The spheres are less able to make bridges than the long grains are and they are like little ball bearings and tend to flow into tight spaces on their own, with the smaller spheres packing the spaces between the large ones. This low propensity to bridge and change bulk density is a primary reason it is much easier to meter uniform charge weights of spherical propellants with a volumetric dispenser (e.g., a powder measure or a scoop).