I have a .177 600mm STX superior on a M3. I heard that STX barrels often don't require much cleaning. My observation is the same as yours. After 100-200 shots I loose accuracy very fast. I shoot a lot and in about 2 months put over 5K pellets through my M3 and the situation was still the same. 95% of my shooting is with JSB, FX or AirArm pellets at 12 - 20 FPE. I followed Orion the Iguana Hunter's video on polishing the barrel. He was even kind enough to call me to walk me through the process before I did it. Ernest Rowe has a video out on polishing the barrel but that protocol requires a lathe. I used a tipton rod, Rolite metal polish that Orion suggests, but did not use the JB Bore cleaner (Orion indicated on the phone that is not necessary). The smooth bore grooves were not aggressive enough to spin the Tipton rod. If you can't find the video on youtube, you can PM me. Also, since I was insecure about making matters worse, Orion suggested that I only polish for 15 minutes instead of 30.
https://www.youtube.com/watch?v=vquzP0dGwN0
Post polishing things have changed for the better, but I'm hopeful for potentially improved results in the future.... Took about 50 shots to regain accuracy. Accuracy is maintained longer, say around 250-300 shots, and when I clean (Patchworm, Ballistol, patches wet then dry) it only takes 2-3 wet patches and 1-2 dry patches. The 1st patch comes out FILTHY. Still need half a magazine (15 shots) for the barrel to settle down.
I probably won't polish anymore until I can find a backup 177 FX replacement barrel. I've inquired everywhere with no luck. If anyone knows of a source, please LMK.
Good luck