Quite A few rifles ended up rebuilt, Primarily Vietnam era rifles. Generally the barrels have been replaced. Some samples of markings uncovered on replacment barrels include: The "C" about the barrel would suggest a chrome chamber only barrel. But AFAIK, Colt marked the "C" up close to the muzzle on https://jareddpsqn.blogproducer.com/37918674/colt-m16a1-for-sale-fundamentals-explained