A preview of the Tokyo Marui Model 870 Gas Powered Tactical Shotgun by RA-Tech as Typhoon Usagi gathered strength in the area, making  the review location a very wet one. They show how to use the gas tank which looks like a magazine, but instead it's inserted in the buttstock to provide the power to propel the BBs outside the shotgun. Since the shotgun shell can be loaded with up to 30 rounds, you don't need to load several shell and only  need one. It is effective up to 30 metres, and they show you can rapidly fire the shotgun in the video.