The OD of the bare barrel (without cover) is 15mm. The front sight fixture sometimes gets stuck on the barrel. Get a small piece of wood and put it against the back of the fixture, parallel to the barrel and give a few light taps with a hammer, it should loosen up and move forward and then be easily removable after that.
BTW, the fellow that was offering you a D54 in .177, a few pages back, did you purchase the 54/.177 from him or were you able to locate one before his offer. I myself looking to get my hands on one. If you have his info please let me have it so I can contact him. Thanks.