The most streamlined AR-15 to ever enter service, the MK18 is a stranger to no-one as it swept the airsoft scene as it did amongst fans of real guns. It’s hard to find space on a MK18 where you CANNOT mount something: the carry handle and front sight block have been snipped and stitched over with as much picatinny rail as will fit. Truly the top dog for CQB situations, and a favourite of US special forces in particular.
 
Metal tooth piston
 
Metal piston head