Popularairsoft has presented an under-barrel sound simulator made in the shape of the HK M320 grenade launcher. The device is available at Taobao.com for around 60 USD.
Visually it looks quite decent. It even has a logo that looks almost like the HK one and the suggestion: "Enjoy Yourself"
There is even a video presenting of the device in action:
Considering the fact that pressing the trigger of the rifle causes flashes in the barrel of the grenade launcher, we decided not to seek more information on this product..
