It looks like the rifling rod to a hand rifling machine.  You put the box with the cutter on the end, clamp the bored barrel in line and push the rod through with the handles.  The cutter follows the spiral.

Then you index the brass nut a partial turn and do it again.  Eventually, you have the correct number of grooves started and you raise the cutter in the rifling head and go around again.  Repeat until the grooves are the proper depth.

The rod looks kind of short.  Maybe for buggy rifles or pistol barrels.