Super easy pen gun. All that you need is a Pilot G2 pen or similar pen. (They have easy access to the spring.)
Step 1: Get a Pilot G2 Pen
Come on people, this step takes five seconds. If you don't have one, you can find them on the ground at school. Once you have obtained this then you can begin with the rest of the steps
Step 2: Disassembly
Disassemble the pen (don't bother with the small white piece at the end of the pen near the clip) That's all you need for this step.
Step 3: Put the Spring on the Writing end of the Pen
Put the spring on the pen.
Step 4: Put the writing part into the "Chamber"
Shove the pen in there- just not too hard. You don't want it in there too hard.
Step 5: Put the cap back on the pen
Put the cap back on- just barely, if you bump it- it will fall off. Then for firing just click the pen and it should get at least 10 - 20 feet depending on how hard you press it. For variation you might want to glue the cap to the end of the writing part.