Worth noting. The upper (where the bolt carrier rides) is still not chromed in mil spec rifles. The barrel is chrome lined. The chambers are therefore chrome lined as well. But the upper is aluminum and is never internally chromed. Thats where you got me a bit confused. Chrome lined barrels do help with durability, but make little difference in reliability. The rifling lasts longer in chrome lined barrels. That doesnt affect function, rather it affects accuracy. Now, bad accuracy can certainly lead to casualty, but not as fast as a jam.