I can't really say from your description in which direction it is loose. If it is in the axial direction, the slop is in the barrel itself (push in-out), then placing a shim behind the circlip will help.
The bracket to which the barrel is mounted has a few (2?) screws, check them as well.
Perhaps even the barrel to the bracket is loose, but I have no clear view on that exact construction, so no specific advice for that, sorry.