We had a Knights job we were firing on a 417 and a Surefire we used on an Aug, both piston driven. The 417 was pretty chocked up after 250 rounds fired at a steady rate. The Aug got to 450-500 before it started choking. They still throw a lot of shit back in the system even with the piston.
Taking the mag off after a serial was a bit of a shock, brass that was shiny before hand looked like it had been lying in the open for a month or two. You get a really thick coating of grey gunge all over the bolt carrier/group. We were only using CLP as a lube as some pogue still thinks its the bees knees.