The no cleaning/lubing a piston gun is a myth that will eventually bite an owner. True the BCG and Bolt stay cleaner (not clean as Erik pointed out), but the carbon and gasses from the firing cycle dont just go away. The piston system still needs to be cleaned of carbon build up and fouling. I end up servicing a number of piston guns (Adams Arms, Sig, PWS and yes even HK) that the owners focused on the same cleaning regimen as a DI gun and disregarded the gas system (piston). The point of gas for the piston gets very hot and are subject to fouling just like a dry bolt in a DI BCG. Shoot a piston gun a ton under nods and look at the areas where the guns get the hottest....it can be very educational.
This is not me bashing piston guns, just a heads up for people that run them. Pistons have a place and can be nicer in some applications, but like other system they need to be maintained properly.
Also I see a ton of people think they dont need to lube their piston gun......dont do it! Keep your bolt/bcg lubed and your gun will run significantly better with less wear on these key parts. Dont believe the marketing BS for coatings and treatments that eliminate the need for lube.
Chris
PS I have run my KAC SR15 multiple time to 4-5000 rds between cleaning, my Noveskes (10.5", 14.7" AND 18")regularly 2-3k rds between cleanings, including supressed. Keep your guns wet.....really wet , even DI guns will run on and on.