No barrel made for ML shooting is safe with steel unless it is so marked. I suspect the Chamber's gun is not marked, so you are going to have to use a plastic shotcup to protect the bore of the gun from the rubbing of the shot, steel, or other non-toxic shot. If its not as soft as lead, it will scratch and gouge your barrel over time, UNLESS you protect the barrel by using heavy plastic shotcups. You can buy good ones from Ballistic's products. If someone else is also making these components, I am not now aware of them. Perhaps someone else knows another source of heavy gauge shotcups, than Ballistics Products.
BTW. When working up a load for your ML shotgun, use regular shot in those plastic shotcups to pattern the load. Once you have a powder and load weight ratio worked out that throws a desired pattern, then you can shoot a little of that expensive non-toxic shot to see how the patterns compare.
Remember, the nice part about shooting Black powder is that it burns slower than smokeless and pushes, rather than slams, the shot forward. This deforms much less shot in the back or bottom end of the shot charge, leaving more round pellets to stay in the pattern down range. With Bismuth shot, for instance, you could expect 20% of the shot at the back of the casing to shatter in to dust and powder before the load left the barrel of the shotgun, so you had that fewer pellets traveling to the target. This is because bismuth was and is rattle brittle, and it cannot stand up to the shock of suddenly being thrust down a barrel under that kind of pressure. Buffers were put into the shot column in the casing to reduce this shattering effect, but the use of black powder actually caused a greater improvment in pellet performance than even the shot buffers. I believe that if the manufacturer had slowed down his velocity on the loaded shells, and just told people that they didn't need all that velocity at the muzzle to kill birds, more people would have bought his shotgun shells.