I just recieved my empire 7 piece barrel kit for my a-5 and i love it. However, im concerned with the lack of a .689 barrel back. The kit goes from .687 to .690. It doesn't really have a bore to go in the middle of these two. Where can I get an individual back that would fit this kit? Would I ever need a .689 back or will the rest of the bores do just fine?