If the choke is marked lead only, NEVER use steel shot in it. Like someone said earlier, steel shot does not compress like lead. Consider Brwoning for example. A modified choke for lead is a full for steel shot, while improved with lead is modified with steel. Bismuth and tungsten polymer can be used in any barrel choke combo, where as Hevi-Shot should only be used in chokes safe for steel. If you are having trouble getting large steel shot to pattern well, you might consider droping down a degree of choke.