Originally the AR-15 / M16 had a Chrome plated Bolt Carrier Group. Later, when adopted by the Military in the Mid-1960's this feature was dropped.
Chrome is significantly easier to clean vs. Phosphate and may have some lubricity advantages when hot.
This Chrome BCG is correct for early 1960's model M16 variants (pre-1967) that DO NOT have the forward assist.
Includes authentic style "machined" Retaining Pin.
The later version with assist serrations is (Here)
158 Carpenter Steel Bolt
8620 Steel Carrier
Compatible with 223, 5.56, 300 blackout
Includes authentic early style "machined" Retaining Pin