I believe that some shot materials that are denser than lead such as steel and Hevi-shot over time will wear on the constriction of the choke itself. This would increase pattern size and decrease choke life. You can buy additional full/extra full/super full turkey chokes that are designed specifically for the denser shot materials. I had to do this myself for my 870 SP to use Hevi-Shot turkey loads when Remington loaded them. I purchased Remingtons Hevi-Shot Super Full choke. The combination was devastating and worth the additional few bucks ($30?) IMO.