This begs the question....what defines a gun as a barn gun.
I should think that the definition or the use of the term
"barn gun" would place the piece near to the ACW if not after it.
I can tell you what qualified for a barn gun in my family, a Remington rimfire rifle kept there to shoot rats. It wasn't well maintained, but it kept shooting and my dad has it today, although it's almost 140 years old now and the rifling is gone. Probably was used for 60 years as the barn gun. I mention it to illustrate the point. IF you're keeping it in the barn it isn't a costly gun, and because of the cost of obtaining and caring for such a gun in the flintlock or caplock eras (prior to industrialization), I'd say the term is ill used for a hand built gun. IF not incorrectly used, I'd say it would be a cheaply made .32 or .36 caplock, or a really small bore, smoothbore caplock perhaps in .40 -.45 and using shot..., but again I don't think "barn gun" is what we're talking about prior to the ACW.
Schimmel is a term that is mentioned in
The Art of Building the Pennsylvania Longrifle . In old German in meant "plain", though today it means "moldy". Except for the presence of a trigger guard, my "PA Mountain Rifle" might pass as such. It has no butt plate nor toe plate, no nose cap, no side plate, a trigger guard and only two thimbles, plus a plain wooden rammer. It was made by Cabin Creek.
http://cabincreek.net/revspecs/pennsylvania-mountain-rifle/ According to the book, the Schimmel was made without a trigger guard, and the buyer would have one put on or fashion one themselves, after purchase.
So if a Schimmel is what one is after, I'd say a very plain cherry stock, medium to darkly stained, no carving, a good lock and a single trigger, with and good barrel in .45 or smaller, with two brass ramrod thimbles made from bending sheet brass around a wooden dowel, plus the simplest of trigger guards (also perhaps from bent brass) would be the answer. Lowest possible parts costs and simplest construction.
(Why the caliber choice? Least expensive to shoot as well as to buy...of course if you're going for deer you need to comply with modern laws. They might've gone after deer with a .38 or even a .36. Most of us can't do that.)
LD