Part 5. Dacron fillers
Modern smokeless powders are progressive in their burning characteristics. They are intended to be ignited at one end and burn through toward the bullet progressively. Smokeless powder does not explode per se. It burns at a predetermined rate. The predictability of pressure curves is relative to the amount of space the powder takes up in the case.
Compressed loads, where the bullet actually presses the powder in when seated, are sometimes acceptable. Very light charges of powder can be VERY dangerous, perhaps more dangerous than too much powder. This is because a very light charge of powder will lay along the side of the case leaving the primer flash hole exposed or partially exposed. When this occurs, the spark of the primer will spread across and not through the powder charge.
Very light charges of fast burning powder are strongly believed to be responsible for detonations and catastrophic failures of numerous revolvers.
To help prevent this phenomenon, many reloaders use a filler to keep the powder charge against the primer flash hole and permit the powder to burn progressively as designed. The most popular of these fillers is a synthetic fiber called Dacron. Dacron does not absorb moisture and seems to not react chemically with the powder.
Dacron is very light with a high degree of loft (it puffs up). A small amount of Dacron, rolled into a ball and placed on top of the powder charge will keep the charge against the primer flash hole. Dacron will also help with the consistency of the load and minimize the standard deviation and extreme spread of the velocities.
Other materials are used as well. Kapok was once popular for this purpose. It is a plant fiber that is extremely flammable and was used in life vests as it would not readily absorb water. Unless you have access to old life vests or life in the tropics, it is not a very viable alternative today.
Cast bullet rifle shooters often use reduced charges of rifle powder or even pistol powders with Dacron fillers to keep the charge oriented properly. For an example, I load .30-06 rifle ammo with a 150 gr rngc cast bullet with a charge of 32 grains of W-748 powder and a Dacron filler weighing 1/10th of a grain. This makes a rolled ball about the size of the last digit of a typical adult finger. This load will shoot 1-1 1/2 groups but will not cycle the action of most semi-auto rifles. This makes for a very economical load that also allows you to manually extract/eject the brass so you do not lose it (on an indoor range or in tall grass). This load also does not tax the brass unduly and stretching after resizing is less than .002. The brass will have a very long life using a light load such as this.
Note: the above load was tested in the authors personal weapon after much experimentation. Use this load data at your own risk.
Never add a filler after you have worked up a load. The filler should always be used in conjunction with the experimentation. Adding a filler to a load will raise the pressures in an unpredictable way. A load that was otherwise safe w/o a filler can become unsafe after the introduction of a filler.
If you are inclined to load very reduced (subsonic) rifle loads with pistol powder, do so with extreme caution. The amount of pistol powder needed to propel a bullet out of a rifle case is very small. It is possible, if not likely you will eventually double charge a load and severely damage the rifle. I know this from personal experience. In searching for an economical reduced load, try to find a powder that will fill at least half of the case volume so any overcharge will be readily apparent.
There are many good ideas and further explanations of this practice in the Cast Bullet Handbook from Lyman. This book has a wealth of knowledge attained over the last 100 or so years of hand loading.