Whitworth tested many, many different combinations of cleaning components. What he came up with was a shell loaded with a paper shot cup full of “free flow” lead, which are tiny little pellets used in radiation shielding. Those are there to provide enough payload weight to boost pressure to around 9600 psi. Underneath the free flow, there are two felt wads and two neoprene rings around a collapsible plastic post. When you shoot the shell, the weight of the free flow shot pushes down, causing the post to spread open and forcing the cleaning wads and neoprene rings against the bore. The first wad cleans the bore and the neoprene ring squeegees it, then the next wad and ring repeat the process.