This is a genuine army issue Blank Firing Adapter (BFA) used in training.
A blank-firing adapter or blank-firing attachment (BFA), sometimes called a blank adapter or blank attachment, is a device used in conjunction with blank ammunition for safety reasons, functional reasons or a combination of them both. Blank firing adapters are required for allowing blanks to cycle most semi-automatic and automatic firearms. It can also be a safety feature designed to break up the (wooden or plastic) plugs replacing the bullet in military blanks (with the added benefit that a live round mistakenly fired will spend most of the energy smashing through the BFA reducing both the range and damage inflicted) as well as divert the hot gases from a blank discharge out to the sides, reducing the risk of injury to the target of an aimed shot.