Public exhibits of longrifles I know:
Colonial Williamsburg, DeWitt Wallace Gallery, ground floor and Masterpiece gallery.
Colonial Williamsburg, Gunshop
Museum of Southern Decorative Arts, Permanent longrifle exhibit
Winterthur, throughout collection.
Smithsonion Museum of American History, on permanent exhibit and in storage.
Virginia Historical Society, Virginia Manufactury of Arms exhibit
Landis Valley Museum, Gunshop and Viewable Storage
State Museum of Pennsylvania, have the best guns from Landis Valley in storage.
Maybe I will think of more later, but the best guns are in private hands.