If the M4 goes to a gas piston impingement system, the operating parts might look something like the long-stroke piston (top) from the FN FNC (Fabrique Nationale Carabine). Gas from the combustion of the cartridge propellant acts on the face of the piston. The FNC uses the same 5.56x45 mm NATO cartridge as the M4. So the dimensions of its piston assembly could be indicative of what might appear in a piston system for the M4, should one be adopted. For comparison is the physically heavier piston assembly from an AK-47 (bottom) which fires 7.62x39 mm cartridges.