The Lewis tool is really just a piece of copper screen with a rubber plug behind it that you pull though the bore.
It does a good job, but I've found that the ChoreBoy copper scouring pad mentioned earlier does as good or better job of getting the lead out of the bore. The Lewis tool, or at least the one that was marketed by Hoppes, also had a forcing cone plug that allowed you to scrubb the lead off of the forcing cone too - which is a good idea.