Step 1: You will Need...
high powered spring-mine was 3.50 from http://www.mcmaster.com. Make sure that the spring is slightly smaller than the crutch so that it can go inside, but you still get maximum power.
Knife (or drumstick if you want to play it safe).. i got mine from china town for 10.
NOTE- in the picture im using a wimpy spring from an old airsoft gun.. didnt get too much distance but the spring i ordered from the link above works beautifully.
Step 2: Get into the "barrel"
Next, insert the spring into the "chamber"
Now take off the rubber tip.
Step 3: Put the knife where the rubber tip used to be
BUUUT! if its too small, you can clamp the metal with a vice grip and fill in some space with hot glue.
Here I am using a drumstick just for demonstration purposes.
Step 4: Aim and Shoot!
Or you can hold it from the hip (ex-how Al Picino, hel phis gun at the ed of Scarface, when he blew up the door)
This is my first published instructable so please be nice haha. Constructive help is always welcome