If the laws of ballistics that apply to rifle cartridges also apply to pistol cartridges, fillers with small charges of smokeless may not be a good idea. The following quotes are from the Spring 2005 issue of Black Powder Cartridge News, an article on chamber ringing by Steve Garbe. His reference was "The Modern Schuetzen Rifle" by Wayne Schwartz.
"A tuft of Dacron fiber weighing about .03 grs was used to hold the powder near the case head with the intent of building pressure to fire form his cases. Using a new case for each shot, we proceeded and got torn necks, split cases and within a few shots a ring started to develop right where the base of the bullet had been. After destroying several cases, we had a significant ring in the barrel."
"We also tried kapok, felt in several thicknesses different quantities of Dacron, and target paper as well as wads made from newspaper. In all cases where the powder was held back against the inside of the case head we developed a ring; loads with Dacron producing the most prominent rings."
"The basic cause of ringing is that a coherent column of high velocity gas and burning grs of powder with considerable kinetic energy, when hitting the base of a stationary bullet, has to stop and in the process generates a thin ring of very high static pressure under the well known law regarding the conservation of energy."
"Experiences reported by other shooters indicated that a charge of loose powder did not ring chambers whereas the same load restrained by a wad against the powder face would sometimes ring and that old barrels were more prone to damage that barrels made of commonly used modern steel."
"The most commonly used filler is corn meal or similar cereal. I have never used this concept as over the years, I have seen several guns that have had half of the cartridge case pulled somewhere up the barrel from the chamber. The cases were pulled apart just about where the filler and powder interface had been."
The author conducted further tests using no filler, holding the rifle vertically. "This test confirmed our view that the ringing was not a result of the wad impacting on the bullet base causing it to enlarge, and so ring the barrel." He states:
"So much for the wads themselves being the culprit. Further test firing straight down produced no sign of a ring."
Reading this article was enough to convince me that I never wanted to experiment with loads using wads and small powder charges in ANY gun. Modern gun steels may well withstand the pressures generated, but I can live without knowing ......