I wasn't planning to post this so soon, Because i was going to wait for KILLERK's MG. But i thought it would be cool to show you people my biggest baddest gun early =P Merry Christmas!
It uses a bullet-type design i made 4 years ago and i put them all on a chain; so it can shoot as many times as you want, it just takes a while to make more bullets. The one i made shoots about 30 Ft at 10 shots per second and has 40 "bullets" on its chain.

Sorry i don't have detailed instructions. =(

Step 1: The Bullets, Chain, and Gun.

1. Here's a picture of one bullet. (unloaded)

2. A picture with the chain off the gun and the gun next to it. (Sorry it's so dark)

3. A front view of the gun

Okay to build the bullet you need:
3 blue rods
1 green rod
1 red connector
1 light grey connector
1 black connector / grey connector
1 chain link
5 grey connectors

From my picture, there is only one piece you can't see, and that's a blue rod. And basically everything is connected to it how it shows on the picture.