He did not take barns by storm, as a soldier might; he merely did his storming in barns, his furious dramatic declamations.
He was an actor, that is, or one who would be.
He was one of the large number of second-rate itinerant players who roamed the countryside in bands, giving a play wherever they could attract an audience.
Dickens describes such a troupe in Nicholas Nickleby. Whenever a proper theater was not available, they made shift in a barn.
It was not until about the mid-nineteenth century, however, that barnstormer was applied to such an actor.
Late in the century the term was also applied to any American political speaker making a rapid campaign tour to arouse the electorate in his behalf.