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.
Step 2: Air Tank
air tanks just 50mm pvc about 40cm long, i put in a small ball valve and a air compressor quick connect to fill it up to a maxium of 120 psi
Step 3: Trigger
the trigger is a blow gun screwed into the air tank and the barrel, i couldn't find a fitting to connect it to the barrel so i used epoxy to glue it into a nipple that i screwed into the barrel.