Alex Pro Firearms (APF) is proud to introduce BCGs featuring SP4, a hybrid DLC (Diamond Like Carbon) coating that is the result of over a year in development. SP4 combines high hardness (2500-3500 HV) with ductility to remain tough without exhibiting brittleness vs common PVD/DLC type thin film coatings. SP4 reduces friction and improves wear resistance for metal components which provides a longer life span and requires minimal maintenance with little to no lubrication. It has been tested to -44 degrees F in the mountains of Alaska, and has been tested in the both Iraq and Afghanistan with no lubrication, which is a huge advantage in desert environments and extreme cold. SP4 has passed the ASTM 85 (Annex 1) Salt Fog test and provides the best hardness/lowest friction co-efficient while maintaining the ability to “move” with the metal it is protecting. Includes a hand polished bolt carrier with a chrome lined bore and staked gas key, bolt with gas rings, firing pin, cam pin, and firing pin retaining pin.