I took my 300S Junior to a local gunsmith for a service back in July, when it became clear that the piston buffer had rotted. Not having heard anything back from him, I called in yesterday. The owner said he had looked at it on the rack every week, and could not pluck up the courage to attempt it. He had only worked on Czech Slavias before. I brought it home and, with the help of two videos on Youtube, managed to do the job myself.
I have no significant previous experience; all I've done before was basic lubrication and spring changes on a BSA Cadet, Original/Diana 23, and HW35. I used the standard kit from Waffencenter Gotha.
I highly recommend following these two videos from Joe W Rhea:
Feinwerkbau 300S Rebuild and assembly
Tools: hollow ground screwdrivers, adjustable spanner, pliers (I used those on my multi-tool, but needle-nose would be better).
I followed the video, and found the strip to be easy. Removing the breech seal was easy, as was scraping out the rotten buffer.
Problems: I broke the new piston ring while trying to fit it; my stupidity. Fortunately, the old one appeared intact so I reused that. Compressing the springs to fit everything back together was hard work and resulted in ten minutes of straining and swearing. Despite taking care, I lost a circlip; thankfully I found it two minutes later, stuck to the bottom of my sock.
Ultimately, it was not nearly as difficult as I had expected. It took two hours, including repeatedly watching sections of the videos to make sure I got things right. The first five shots dieseled, but the following ten came out fine. It seems to have been a success.
