Piston system. HK MR223 without the hand guard, showing what is inside of it. The system consists of black rod, that returns to battery with a spring and the piston, shown in another picture. The handguard attaches with a screw and around 7Nm of torque.

HK gas piston, the part that takes the pressure from the barrel and turns it into movement. The maintenance book advises to oil it but I am not so sure, it seems to do just the same no matter how it looks. It takes some serious carbon caking on it but it does not seem to have any effect at all on it

Shows 2 ball bearing screws used to tighten the movement of the stock to minimum. They indeed work well.