I have a full size P5 with an extended threaded barrel. The barrel was extened by welding, and then turned and threaded. To handle it, you would never know it was not a long factory barrel without noticing the short unrifled section at the muzzle. Unfortunately, the man who did this for me is retired and not interested in doing this type of work anymore. Any good gusmith with welding skill should be able to perform a similar operation for you.