In essence, a cannon is a strong tube (called the barrel) that is sealed off at one end (the breach). An explosion at the sealed end forces the hot gasses up and out of the muzzle, pushing whatever is in front of it out at a tremendous (we hope) velocity. There are few design choices and explosives you can use, effectively, but we'll discover those as the episode progresses.
 
 
 

The Chemical Brothers l to r: Pat, Captain Paul and Rob David Rainer is their expert on the bottom	The (mustachioed) Brothers in Arms David, Dick and team captain Bobby Strawbridge inset; Dr Derek Allsop
As you can see, the explosion at the back of the cannon will cause the steel to bulge momentarily before expunging the projectile	This monsterous cannon was used by the Germans during WWII. It's mounted on a train!
The Chemical Brothers will be using age-old black powder.	The Brothers In Arms will be using cordite, a modern explosive.
Our judge today is Major Dave Reid who is a weapons tester.	Derek? Did you remember to carry the four?
While the Chemical Brothers are readying a sight for their cannon...	...Brothers In Arms expert Derek is being quite lathe-y.