Small steel shot (6s) should not be any harder on a choke than any HTL material (HS HW TSS). But I'm pretty sure you will void the warranty on most chokes if you do so and damage the choke. Large steel shot should never ever be used in a tightly constricted turkey choke due to bridging and the strong likelihood of choke damage. Besides, steel is the poorest ballistic choice possible for killing anything. Very bad idea.