It seems to be resting on a stand, which would indicate it may not have been hand held. It has what appears to be a circular saw device, and a firing pin.Well still clueless, it should be a gun for shooting attached to a saw for butchering shot prey, device thingymajig.That does not appear to be one of the options though, curious.
I vote for gunpowder tester, the pin fires and causes the saw to spin round, which then causes sparks,a bit like a flint and if it all goes BOOM !! I guess the gunpowder is not wet.
It is on a stand so you can pull the trigger and run like hell.
Yes I know, time to go off with the nice little men in white coats again. I need to get a job, I'm really not fit to be left to my own devices for so much of the day. !!
Hah, just thought, it casts a whole new slant on the, "gun's goin off" of the S.S.