SpudFiles

I'm new to all this, and could you tell me if I could make a BBMG by just putting a line from an air compressor on one end of the barrel, have a valve between that and a vertical gravity-fed magazine? If the barrel was roughly the size of a BB, only one would fit in at a time, correct? I'd really appreciate the help.

It may work, but why not make a cloud bbmg. Its extremely simple, and has a pretty good rate of fire. Heres how they work:
You attach an air compressor blowgun to a small pvc chamber which is filled with bb's. At the other end of the chamber is a barrel (usually made of brakeline). When the trigger of the blowgun is pressed, the stream of air agitates the bb's inside the chamber. They bounce around until they get close enough to the barrel to be blown through and fired out the end. It is a very simple and effective bb machine gun design.

Thanks for the tip!

that design probably wont work because there is to much air leaking around the bb. also dont build a cloud build an inline vortex they are way more efficent.

Its your gun mate cloud or inline just have a look through the showcase section and pick a design you like and go for it. Good luck mate