My hammer double rifles are all rather old. The most intriguing is a military double rifle issued to Austrian riflemen of the Border Guards (a special light infantry unit) in the late 18th century. These rifles are OU's of incredible construction. It is a rare and remarkable piece that I have not had the courage fire.
I have two Jaeger double rifles from mid-19th century Germany. Ross Seyfried suggested loads which regulated beautifully. Both took whitetail in Virginia. The second is particularly exquisite.
Shotguns are a different matter. Two I have use extensively. The first is a H. A Lindner 2 7/8's inch 10 bore designed for 1 1/4 ounce loads, It has taken a medium size truck load of geese and ducks in Canada and Maryland - often to the stunned surprise of fellow hunters armed with the latest semi's from Benelli, Beretta, or Browning.
The second is a modern built SxS 20 bore by Luchini. Over the last decade and a half it has taken many dozens of quail, doves and pheasants. Unlike vintage hammer guns, the Luchini is equipped with a safety, making it much easier to manage in the field. Just a special thing.
And finally another that I have only used once on a clays range. It is an 1840's era Webley and Scott 12 bore in pristine condition. Balance is as perfect as any modern best gun. This one has it's original mahogany case.