This an improved version of joemonkey's knex stiletto knife.
Step 1: Make the Regular Stiletto
First you have to have joemonkey's regular stiletto knife. It can be found here. https://www.instructables.com/id/knex-stiletto/
Step 2: Add Supports
Pic 1: Make this
Pic 2:Make these (they are two orange connectors with green rods on one end)
Pic 3: Attach pic 2 to pic 1.
Pic 4: attach pic 3 to the knife.
Step 3: New Locking Mech
Pic 1: Get these pieces.
Pic 2: Attach the Y-connectors to the knife (You should replace the white rods that were there with blue rods.)
Pic 3: Add the orange connector.
Pic 4: Attach the rubber band on the side that does not have the orange connector.
Step 4: Locking and "Firing"
To lock it pull the blade down into the gun and the press the white rod down so that the white rod is being held in place by the orange connector, (see pic 1)
To fire press down on the white rod until the blade shoots out, (see pics 2, 3, and 4).