Re: 870 barrel and steel shot?
I would have to guess since it is an 870 that is only chambered for 2 3/4" shells, and it is a fixed mod choke that it is NOT steel shot approved.
If you want to hunt waterfowl with non-toxic shot I would recommend getting on the waterfowl forum, and asking what kinds of shot can be shot from older barrrels. There are some out there. Yes. It will cost more than steel shot, but unless you are doing a lot of shooting it will be cheaper than a new barrel.
Another option would be to trade the gun you have now for a newer model. You could get an express that is rated for steel, have a 3" chamber, and have the option for different choke tubes. I think those guns go for around $300. If you could get $200 for yours it would only cost you $100.
Venison - The other fast food