Ok, the Floss Stahl is literally river steel, taken to mean fluid steel though, ie not damascus. So, pending a gunsmith's check-out it should be ok to shoot. Now, the major consideration is chambering, it is 16ga, but pretty much guaranteed to be 2.5" chambering. In addition, the forcing cones were designed with roll crimp fiber wad shells in mind. So, a modern 2 3/4 16 ga may fit, but it is loaded to a higher pressure spec to begin with, the pressure will increase as the star crimp extends itself into the forcing cone and bad things may happen.
It is possible to have chambers lengthened if the gun has suitable metal. It is also possible that they already lengthened, only the gunsmith will know. Do not fire modern ammo unless you know. Easiest route is Gamebore cartridges produces 16ga 2.5 in shells, I picked some up at Cabelas for my pre-war drilling and my Win 1897 16ga. When I was a noob I fired a box of modern 16ga through the 1897 not knowing any better, the gun and me are still ok, but I'll try to keep it that way and not repeat the experiment.