Just like London busses - you wait years for interesting guns to come along and then two turn up in a space of a few weeks. Rather than start a new thread it seems appropriate to add this one to the Dougall topic as it bears some similarities. There is probably 140 or 150 years between them but just have a look at the photo. You may need to double click the image to get it to a good size as I am now using a different programme from photobucket as it is almost impossible to get it to work.

Well yes it is an O/U and it doesn't drop open but it does slide forward and it does have the bosses on the breach faces like the Dougall. I apologise for the quality of the photo but I only had a few moments and the phone to hand.
I am told by the owner - who has had it 20 years and it certainly wasn't new when he bought it, that it was made as a prototype by a man called Cavener of Cheltenham (UK)It is a double trigger, non ejector. He believes it is a one off. The top lever is pushed down to lift the locking gate then the barrels will slide forward to load/unload the gun. He says that it isn't the slickest of mechanisms!! The triggers are interesting as well - you have to pull the back one before you can pull the front one. The gun will fire if you pull the front trigger exceptionally hard - so he isn't sure if the back trigger unlocks the front or it is rubbish engineering. Once the back trigger has been fired the front one works very smoothly. The firing pins are also interesting - they are spring loaded but to leave them protruding rather than being retracted. As the barrels are pulled back they are pushed in by the cartridges so they are sitting on the cap under a little pressure. Again a novel idea but apparently it has never misfired. A fascinating piece of engineering but one it is easy to see why it didn't get beyond the prototype!
John