I got it figured out. There was some residual trash left on the threads from a previous owners AAC device. I cleaned it up real good and it bottomed out on the shoulder. Then I went and got shims (556) and played the shim game for about an hour and a half. I think whoever makes those shims has a laugh when they ship them out. Because it seems like every time you throw one on it ends up in exactly the wrong position. Finally figured it out though.