Here is a mouse trap catapult designed for safety and economy. We designed and built it for our project management class. It has a simple design and is easy to construct (we built it in about ten minutes). It has a healthy range for it's size and is fun to fire.

Step 1: Parts Needed

One mouse trap--<$1
Eight cheep white rubber erasers--$1
One roll duct tape--$1
Plastic utensils --you probably have them already, one knife, one spoon

The erasers are just for absorbing the impact of the mouse trap bar so that the catapult will throw projectiles in the right direction. There are probably many options for substitutions.
Any normal mousetrap should be adequate, the same goes for the plasticware.
Use only the finest duct tape.

Step 2: Safety First

After removing the bait mount and trigger rod, bend the snapper (main bar that would hit the mouse) as far as it will go and secure it with duct tape. This will protect your fingers and materials.

Step 3: The Rubber Wall

Place the erasers on the empty side of the trap and stack them so they reach high enough and deep enough to absorb the blow from the snapper. Secure it with plenty of duct tape. The erasers will hold up well under the crushing blows, and the duct tape will be invincible.

Step 4: There Is a Spoon

Release the snapper (carefully). You can test the eraser attachment with a few snaps from the snapper.
Put the flat part of the plastic knife across the snapper--this will act as a solid anchor to tape the spoon to and increase the durability of the catapult.
Place the handle part of the knife behind the spoon to support it as it flings projectiles at your enemies. Secure all parts with duct tape.

Step 5: Conquer the World

Your final product should look like ours--very nice.
The video show how far it will throw a rolled marshmallow. Pennies went farther but are more dangerous.

Conquer with care.