Accurate Ball Launcher PVC

In my physics classroom we study projectile motion. Accurate projectile launchers with consistent results are very expensive ($200+). Because of this, I find that I have many students working with few launchers, diminishing the overall experience.

I have designed a launcher that I have been able to create for my classroom that costs less than $5 in materials. Aside from plastic, you will need a large spring ($0.80, I used menards part#88228, 13/16 x 3-1/4 compression spring), and a 5 inch section of 1" schedule 40 PVC pipe.

To launch, a paperclip is used to hold the plunger back. A swift yank will release the plunger, and your projectile should be launched. I used a 1/2" ball bearning, and I found that the projectile would travel around 11 feet, with an error of around 6 inches.

Some of the troubles I ran into, that I expect you will also find:
-PVC sizing is not as consistent as I initially thought. I had to adjust the scaling percentage slightly depending on the batch of P