DISCLAIMER: I am not responsible for anything that happens with your cannon. Build quality and material quality is not always the same. Some laws prohibit the use of firecrackers in certain areas. Check all state and local laws before using firecrackers.
Step 1: Materials
1 - 1/4"X2" iron pipe nipple
1 - 1/4"X1/4" coupler
1 - 1/4" air quick connect fitting
All materials except the firecrackers can be found at a local hardware store.
Firecrackers can be found at a fireworks stand or store around the 4th of July, so around that time, STOCK UP.