I had the same scoring on the gas tube in my NPSS in just about the same place. I think this was one of the causes of the grinding I felt when cocking. I lightly filed the inside of the rectangular slot at the end of the piston with a small diamond encrusted file (that's pretty tough steel) continuously checking to the point where you couldn't feel any drag on your finger. Also, in my case, the cocking shoe face was mangled up (meaning the face which should be flat was not flat). The cocking arm shoe took some pretty heavy filing to get the face flat, then 340 grit emery cloth to get it smooth, then polishing with jewelers rouge. I spent some time with the round file on the concave surfaces of the cocking shoe also as they were gurred up a little also. Lastly, I spent quite a bit of time on the cocking slot just south of the compression chamber. The edges of the slot were sharp and the slot itself (not the edges) was really rough....looked like the surface of the moon with bumps and valleys. I spent several minutes with a small wood sanding block (the size of a tongue depressor) and 340 grit emery cloth sanding it smooth. While it was all apart, I polished the trigger sear that snags the piston as well. After all that sanding and polishing, I cleaned everything with de-natured alcohol. I lubed with silicone and moly per CDT's guidance paying attention to the gas piston where the scoring was and re-assembled. I didn't sand/polish the compression chamber or the inside of the piston like Gene because they looked pretty smooth already. After re-assembly and now after about 200 shots, I can profess that I am really happy that I took it apart and worked it over. This is a truely fine rifle and being motivated by Gene and educated what to look for and how to fix minor problems, it's even better than it was before. So if you're sitting on the fence debating about doing a Lube Tune on your NPSS....debate no more, tear it down and work it over. When you put it all back together with the right lubes in the right places, you will be extremely pleased...not only that the shooting cycle is smoother and now more accurate, but that you know more about how your rifle works and what makes it work better. Oooooh RahhHHH! Semper Fi Tom