I first ran into the term "
barrel" awhile back in some
Hall stage lit. They called hardware, "fixings" and pipes/battens "barrels". Since then, I've heard it used several more times. Seems to be completely interchangeable with pipe or
batten. I'd say the difference between a pipe, a
batten, and a
barrel is...zip, nadda.
Download some curtain
track cut sheets from thernstage.com. You can see the term "
barrel" in action.
I have heard the term
barrel or
barrel stop used for a
NiCopress stop sleeve too, but only as a layman's term, not by a manufacturer.