Items needed:
Petg- 1/2 And you barrel. (mine is 8 inch.)
Pvc- I use 12 inches, to hold 2 1.75 inch darts with some space.
Plumbers goop
Electrical tape
Hot glue
To start, make a speed loader. (you should know how to, you just connect the two pvc pipes together with hot glue with a inch off of each end and then put e-tape over it.)

Next, cut your petg to the length of your barrel.

And plumbers goop in it.

Then, cut a small 1/2 peice of petg,

And plumbers goop in in behind the barrel you glued in.

Then repeat all the steps and wait 24-48 hours for the goop to dry.
Here is a test video.

As you can see, this provides a short 2 dart rapid fire burst, and then you flip the speed loader and have another burst. and then reload. I am still having some issues with it, such as the dart not always going very far, or shooting two darts at once. With my crossbow, it gets 90 instead of 100. I think it is worth it.
-struck by stefan