[Ron Wood]

I have never used a penney but if you flatten the mouth of a shell case it gives you a nice square brass scraping tool that can be used on the heavier rust scale. Also, just as suggested with the steel wool, use a generous amount of oil when scraping which will keep the scale in suspension and make it easier to wipe of as you go. The copper or brass will leave a bit of color on the gun but that is mainly on the rust part and will come right off with oil and 0000 steel wool.