I don't see any wear on your piston, just copper buildup.
ETA: Just found this. Looks to be normal for a PWS 300 BLK SBR:
http://www.thetruthaboutguns.com/2014/11/tyler-kee/gun-review-pws-mk-109/
"However, when I it came to the business end of the piston system, I was shocked at all the crud I found.
I assumed that I was dealing with some baked-on carbon, not unexpected given the operating conditions. But after much soaking and scrubbing with KG1, I found a much more insidious buildup of copper. Lots of it.
I’ll leave it up to our gunsmiths and technical wizards to tell me how that much copper accumulated on the end of the piston, but what I can tell you is that 20 minutes later, panting from exertion (mostly joking), I had finally managed to scrape away the copper leaving a clean piston. If you buy a MK109, I’d suggest you keep tabs on the end of the piston to see what your buildup looks like. It never seemed to affect the function of the gun, but it was certainly an oddity of sorts."