I think Carpmaster was on the right track advising you away from using tight chokes with steel shot. The risk is that the barrel will first stretch around the area of the choke as the high confinement of a tight choke creates a drag on the passage of the shot out the end of the barrel (lead shot will deform, reducing this drag effect, steel shot will not deform). Once stretched, the barrel is then weakened at that point and is subject to bulging in response to interior gas pressure. Thus, the common advice is DO NOT USE FULL CHOKE WITH STEEL SHOT.

Call Remington about whether your 870 is steel shot enabled. Have your serial numbers looked up and ready to hand when you call.