Wait a minute...
Where would the Sulfur and Potassium Chloride come from?
Black bowder guns would leave sulfur residue, but I'm not aware of sulfur being an ingredient in any modern "smokeless" RIMFIRE propellants (to avoid confusion with pyrodex - a so-called smokeless propellant, but not used in rimfires)
As for the potassium chloride...well...maybe, if the ammo manufacturer adds potassium chlorate to the primer mix, which is usually lead styphenate and finely-ground glass (to aid with ignition)
I would think the black residue would be deposits of carbon from the combustion of the propellant, which consists of a mixture of Nitrocellulose, Nitroglycerine and Dimethylaniline (as a stabilizer) along with other organic additives to enhance combustion, stability and cleanliness. When any organic compound burns, some of the combustion takes place "completely" that is, the material is reduced to water vapor and carbon. It's not a huge percentage, but some.
In addition to the carbon, there is lead from the primer and shavings from the bullet itself.
Add that to unburnt powder residue - keeps companies like Hoppe's in business.