Piston, nozzle, cylinder, cylinder head, and barrel are the main ones. I have been wondering what the actual inner diameter of the barrel is, since sites says everything from 6.03 to 6.01 seemingly at random. I threw a PDI W-hold into my Arcturus with no other mods (the stock hop rubber wasn’t quite getting me the range I needed), and the accuracy was truly excellent, which leads me to believe that the nozzle and barrel are both very high quality.