It's called a sub base (also called a sub bridge). The bullet is seated on top of it. It helps in creating a better gas seal with these harder brass bullets since they don't obturate very much (if at all). I got mine from MMP. I think Midway carries them as well. I tried them under some of my connies and really saw little to no difference in performance but Sabot seems to feel they do help greatly with these brass bullets. I'll be using them to try out my new bloodlines when (if) they ever get here.