OLYMPUS DIGITAL CAMERA April 15, 2013 at: 520 × 361 In: M27 First Impressions ← Previous Next → Buffer and buffer spring. The buffer tube has three vent ports at the rear and the buffer is marked HK on the tube face with a red dot in the center as well. The buffer spring is marked with red to distinguish it from a M16A4 or M4 buffer spring.