Probably already addressed, but- no, that M240B is equipped with a Blank Firing Adapter (BFA- the orange item at the muzzle), and is firing blank cartridges. Basically it is a plug at the muzzle with a small vent, which allows the blank round to pressurize the bore/gas system, operating the mechanism as if it was firing a round with a bullet. The flash you see is the vented propellant gases.
Thanks for the explanation.