answer:Having a longer barrel would reduce noise. Whether or not that will effect the trajectory and speed would have to be tested. If you plan on building something as you described be sure that you are wearing eye protection when testing, and that people and animals are a safe distance away. In regards to creating a removable silencer, be sure you understand that just possessing one, even a home made one for use on an air cannon- disposable or not- can get you into a bit of trouble. This short read explains how seriously ATF takes any type of silencer. Sure you are not likely to be caught, however a stroke of bad luck can make you wish you had not attempted it at all.