In my experience the double gun market is currently Very Soft and many dealers are asking price the might have gotten years ago, especially Brit guns because 90% on them were exported to America in the past couple of decades. There are many more guns than people that want them. I would consider the current state a buyers market and deal accordingly.
I have owned basic Birmingham type guns for well over a decade and am convinced that their values have not increased one penny. In fact, they've probably declined. Not a very attractive investment. Over the past year I've bought a couple at prices much lower than I would have dreamed of just a few years ago.
