During the 1970s and 1980s, Colt wanted to sell not just standard M16 rifle and CAR-15 carbines to foreign military customers, but also wanted to supply support weapons. They put together the Model 621, aka M16A1 “HBAR” (Heavy Barreled Assault Rifle - not the same idea and the HBAR target rifles much more commonly seen) as a sort of LMG sister to the M16A1 rifle. It was essentially the exact same idea as the RPK - a support machine gun with parts interchangeability and identical handling to the standard infantry rifle. A small number were made in the 9 million serial number range, but no contracts were ever made to sell them in significant numbers. A few, like this one, eventually came out of Colt and into the civilian collectors’ market.