New and improved bolt assemblies for M4M16 firearms capable of firing a remarkably greater number of rounds than previously available M4M16 firearms are, disclosed that have right and left compression springs that are parallel to each other, straddle the firing pin bore and are retained in partial bores that have proximal and distal ramps descending from their open ends. The error amplifier inputs a voltage signal corresponding to an output voltage of a DCDC result and a plurality of reference voltage signals. An attachment mechanism may include a capture feature shaped to receive the second end of the inferior strut, and the mechanism may provide an adjustable configuration, allowing polyaxial adjustment between the inferior articular surface and the second end. The reflective layer includes a light-transmissive interference layer 14, and the interference layer 14 each has an optical layer thickness of 100 to 600 nm so as to generate an interference color having a visible light wavelength depending on the incident angle of light.