Never a real one.
My neighbor built one by scaling off of photographs in a magazine article I gave him.
A Mountain Man speaker at a past NRA convention had one built as a copy of a museum rifle, I think Jim Bridger's.
I am sure they served their purpose, but the first time I picked up a 7 lb fowling piece, I kind of lost interest in the heavy old rifles.
Without eyeglasses, I am not effective with an open sighted rifle anyhow. If I had lived in frontier days, I would have been better armed with fowling piece, fusil, musket, or shotgun. And a lot less burdened.