I know that a lot of the fellows here prefer Dacron, because Ross says...but from my tests I have come to prefer Kapok.

My loading involving fillers is centered around loading NfB loads in the Fat Boy 577/450 Martini case. It is a big bodied bottle necked case.

The various foams wouldn't fill the body so the were not considered.

Dacron worked just fine, at least it looked like it did. Then I tried kapok.

First thing I noticed was down range debris from kapok was minimal compared to dacron. Recovered pieces of dacron were quite large, sometimes appearing to be no loss of material. Dacron debris was always found after testing no matter how little was used.

Large clumps of Kapok were none existance. As I write this I don't recall finding any. Small volumes of 1 to 3 grains kapok never resulted in any debris being found after firing. When I got into the area of heavily compressed volumes of 5 to 7 grains kapok, debris did show up down range floating in the air.

Here is a picture we took to show it. That's not snow falling, that's kapok!!

At the time I just figured the kapok was being blown apart by the muzzle blast and the Dacron was not.

While working on loads I got some hang fires and noticed that even though the powder hadn't ignited the kapok appeared burned.

I wanted to do some sort of test to try and show that the kapok was being burned. I loaded a primed case as full as I could get with kapok, topped it off with a wax wad and fired it. (Firing only caused the wad to protrude from the case.)

This close up shows the burned and unburned fibers. You can see the long unburned fibers quite well. The burned fibers are the black dots. I also noticed that the fibers appeared broken up or shattered into a fine powder. All this burned material came from a primer only.

I believe the kapok is being burned and the fibers being shattered then expelled out the barrel when fired. This is why I never found any large clumps of kapok down range. I didn't test any Dacron in this manner as by this time I had pretty much eliminated it from consideration.

As side note I also used Kapok as a filler for blackpowder loads in the Martini and found as I used greater and greater volumes my groups got smaller. I think I was getting some moderate benefit from the compression of the powder by the compression of the filler. That is something that is difficult to do with blackpowder in a bottleneck cartridge.

Where to get Kapok. Try upholstery or furniture repair shops. I also have a supplier listed on my Internet Sites for Martinis, Related Material and Subjects website. Here that supplier is:
Kapok - http://www.atrim.com/Page11.html
They offer 10 lb bags for $35 USD. You can buy smaller quatities from E-Bay but it is more expensive.