-for .25. Don't know that he can help w/ an RWS guns, but he has changed barrels on GAMOs, Crosmans, etc.
http://home.comcast.net/~peterdragin/index.html/
For .22, probably easier/cheaper to scrounge a used one in the breech block. I understand the 34 and 350 series are interchangeable, but have not persoanally tried this.
The receiver is the same for both .177 and .22 calibers. Opening the port a bit for the .25 might be a workable idea, but you'll want to check w/ Rich on this aspect.