I have tried similar and it will always happen like that, there is too much air just to launch one BB, it will try to launch the next one or two at a reduced velocity, so they just roll down the barrel. If you want semi auto you need to block the magazine tube somehow between each shot, that's the t...

Can anyone remember a movie plot (or TV show?) where some people break into a facility with a computer virus to load into a super computer to stop it taking over the world (as they tend to do) only to upload it and realize the virus actually triggered the super computer into action.