i just made this cloud BB MG, its fully portable and easy to operate. i got the idea from nightharkinlight https://www.youtube.com/watch?v=ME1BcEcBD9I you should look at this cos it shows how to build it well. i just modified the design
Step 1: the barrel
the barrel is 6 mm, I used a aluminium arrow shaft. the bbs go in the back and the air in the bottem. when the air flows through the bbs bounce around and fire out at a high fire rate.