Side lock with a REMOVABLE breech plug / lug.
What is the point?
I really don't know what a 'brush' gun is. By my definition it is one you carry hunting in thick woods and brush.
For almost a half century I carried a flintlock rifle with a 42" barrel in all kinds of thick woods and brush.
Ded deer didn't know I wasn't using pellets or didn't have a removable breech plug or had a short barrel. They wuz jest ded.