I can't authenticate this offhand, and maybe somebody can chime in with a more factual answer. It's my understanding that "schimmel" means something old and dusty left in the barn. Somehow in our more modern times the frontstuffer community has decided to call old simple guns "barn guns".
I suppose many old basic guns have been found in barns or elsewhere collecting dust.
Anyway, there were probably plenty of basic guns built with not much more than the essential "lock, stock and barrel". Sheezamm, A barn gun!
So, if that's the making of a barn gun then the formula would not be hard to follow.