Please clarify something for me. What is the difference in a shimmel, a barn gun and a po-boy? Aren't they really the same thing?
They are words and words are used differently by different people.
"Schimmel" is a word used first by Chuck Dixon to describe a type of plain, early 1800's rifles found in and around the Reading area of Pennsylvania. These often had just one or two ramrod thimbles, might or might not have a buttplate, and might have a strap metal guard instead of a nice rifle or fowler guard. So you see this term applies to guns from one period and place, and not to plain guns made in the South etc.
"Barn gun" is another term for guns like I described above, but geographically might extend beyond a very small part of Pennsylyvania and also might extend across a greater time span. It's a little more general, but not appropriate for a plain, simple Southern rifle in my opinion.
"Poor-boy" is a buckskinner term for a Southern rifle with forged iron mounts. It might or might not lack some of the parts we're used to seeing.
Some makers use these terms indiscriminately, adding to my confusion. These are my working definitions; others may vary because words are used differently by different people in different places and at different times. As far as I know, none of these terms were ever used in the original flintlock period.