The gun has been completely disassembled and cleaned of cosmoline and all other types of debris. It is probably just a worn out sight. The gears look alright but I don't know maybe they are too worn to click properly. There is a little play in the front gas tube where the barrel and gas tube meet but not much.May have dirt or cosmoline in it, or is worn out. Detail strip the rear sight and see what you have.
Also, any play associated with the front gas tube on the barrel splines?
Thanks.