Hi Evert
This may be of use to you :-
http://www.airgunbbs.com/showthread....ht=Webley+hawk
As the hawk is the same bore and stroke as the Osprey the information could help you, I found a 2.5mm transfer port to work well on the Hawk.
What length is the transfer port on the Osprey as that needs taking into account when sleeving down ?
All the best Mick