I shoot steel all the time out of an old 870 fixed full choke 30" barrel never had a problem. But u are taking a chance of bubbling the barrel or splitting the barrel at the choke. I say shoot it I do it all the time.
I was gunsmithing in the 70's and 80's and early 90's in Minnesota (where everyone is a duck hunter). I worked with two large gun shops near me plus my own clientele. This was when steel just came out and was made poorly as the ammo companies were learning as they went. I never seen or heard 1st hand accounts of any guns being damaged by steel. I heard alot of people say " I heard this or that happens when you shoot steel" but no 1st hand accounts. It was great for business as I opened many full chokes up to modified. I shot alot of steel thru my old 870 full choke.