We get this question a lot. A lot of the "loudness" has to do with how tight you pack your wadding. Even a smaller cannon can let of a comparable "boom" to a bigger cannon. That being said, the "loudest" cannon is most likely going to be the "Fat Boy." See the chart that below shows decibel readings in controlled "loudness" tests. Keep in mind that a fraction of a decimal point is still a significant change in perceived loudness. Every 3dB is a doubling in sound pressure.
https://static.wixstatic.com/media/42b1a4_d8544896b86d405fbd38d1b6f1c55f28~mv2.png
