Really no exact criteria for filler use other than anywhere around a half full case is getting close to at least the potential for causing an unexpected and unpredictable explosive event (also called a SEE- secondary explosive event) ... or something like that. The theory is split in that there are two possible causes. One is that the primer flashes over the top of the partially filled case and exposes a bunch of powder to ignition all at once causing an explosive event or it ignites the charge at the bullet first and the charge burns to the rear creating a wave of compression that affects the burn rate of the progressive smokeless charge causing an unusually sharp pressure curve. The second is that with some especially hard to ignite slow ball powders, if not directly exposed to the primer charge as with the half full case example, may not ignite very quickly allowing the primer impulse to move the bullet into a hard jam in the throat whereupon it becomes a "bore obstruction" causing a huge pressure spike when the main charge finally ignites and comes up to full progressive burn. This stop-start-stop phenomenon is also known where the primer impulse pushes a bullet forward jamming it into the forcing cone of a revolver before full ignition of the main charge... especially with certain hard to ignite ball powders in cold conditions.
A filler is of such low density, that if done properly, it adds nearly no mass to the overall projectile and simply holds the powder charge next to the primer for consistent, full ignition-- especially in large volume cases where even an appropriate powder charge may be far short of a 100% load density. The idea is to use an amount of low density Dacron fiber filler to fill the entire air space, with a small amount of compression, between the bullet base and the powder charge. Just dense enough to hold the powder granules securely to the rear. Some loaders even use a low density foam plug between powder and bullet but I have no experience with this method. Dacron filler is considered superior to kapok, cotton, wool and such as these materials may compress and become a loose, separate, more dense mass in the case- not good! Any hard, dense object sitting, separately on top of a charge of smokeless can be rammed into the bullet base upon main charge ignition with such force that is can cause a ring in the chamber at the place the bullet's base is sitting in the loaded round. not good!
Also, the use of low density fillers is
not the same as the use of over powder hard card or fiber wads. Two different things completely and
cannot be interchanged in application. Hard fiber and card wads are for use in shotshells and for certain blackpowder cartridge loads where 100% load density exists.
No matter the filler- use caution and have those with knowledge of the proper use in any specific application, caliber, powder or charge... guide the proper use directly.
