Singular versus plural. I'm just sayin' technically....
Ground walnut is a medium.
Corncob is a medium.
Ceramic grit is a medium.
Walnut, corncob and ceramic are media for cleaning. Grammar uses the plural when we're talking about more than one medium.
Example:
Television is a communications medium, even though there are many television stations.
Radio is a communications medium, even though there are many radio stations.
Newspaper is a communications medium, composed of many different newspapers.
Together they are the media.
I suppose we get confused about the plural because a tumbler full of ground corncob contains thousands of particles. But similarly, the atmosphere contains thousands of particles, yet we don't pluralize it. Same with the Atlantic Ocean, a body of particles but we don't pluralize Atlantic Oceans.
On the other hand, we do enjoy a bowl full of grits or corn flakes. So we can pluralize a bowl full of walnut or corncob without being wrong. At least in the reloading world, we all know what we mean by tumbling media.