Included in this kit are the small items needed to make your Hand Cannon prop able to have realistic blowback and shell ejection. 
 
What's in the package?:
1x - M3 x 0.5 - 16mm Slot Head Screw
1x - M3 x 0.5 Heat-set Threaded Insert
2x - Size 19 Large Rubber Bands
2x - 1/4 inch Orthodotic Rubber Bands
3x - 1 inch Rubber Bands
4x - Standard Size Paper Clips
 
To buy all of these parts separately in bulk would cost over $20 via Amazon.
 
The 3D Printing files that go along with this kit can be found here: 
www.theraygunproject.com/projects/handcannon