This isn't really an amazing mod as everyone probably knows it.
What you will need:
A stretchy elastic band or two.
A nerf gun pistol with a pull back
Step 1: The elastic band
1-Place the two bands so that they are overlapping each other.
2-Pull the end of the band on the top under the edge of the band on the bottom.
3-pull the end that you have put under the other band back up through the loop and pull tight.
If you don't understand then look at the pictures.