Be careful of your use of the term "barn gun", I have been burnt on this forum with its use. It appears you are more interested in what can be done in the way of building a smoothbore that a person of lesser means would have, more than a gun to shoot barns with as some would comment on. I also built a smoothbore in '99 along the lines you are looking at, except I wasn't as informed as yourself, nor did I communicate with others on the period correctness of it. My fowler has an English lock and trigger guard, Germanic rifle style buttplate (though rounded at the bottom), a French s-shape side plate, fancy ramrod pipes, and otherwise plain. My idea was a gun made up of pieces from worn out firearms a gunsmith would have available to build for a person of lesser means. Some parts I made myself like the trigger and side plate. It has a crudeness about it that others may not approve of, wanting to have and see only the prettiest of guns. However, look at the link below of some original American Revolutionary War smoothbores. Some look even more crude than one I built. I ran across this web site in the last year and feel vindicated that what I built isn't far off the mark for period correctness.
http://www.11thpa.org/neumann.html[/quote]
This book:
FLINTLOCK FOWLERS
Has nothing but North American Colinial fowlers in it, most made from recycled parts. Some are very nice, some are not so nice.