This is way ambitious but you can actually make your own 25MM Pneumatic Sniper! This homemade weapon is made entirely out of PVC pipes that can be bought in your local hardware store. Word of caution though before starting the project. This DIY can threaten or harm anyone and you need to be very careful in making this device. In line with this, you also need to do some research if this device is allowed in your state.
On the next page are steps for building your own 25MM Pneumatic Sniper Rifle using PVC.