The reduced chamber I have no experience with, so I can't comment there.
As far as the cardboard, my guess is that it pushes against the air on it's way out (like a parachute) and provides a slight but uniform resistantance against the burning powder. It weighs almost nothing so it can't be the weight.
I think it keeps some of the the powder from leaving the barrel too soon. It makes quite a bit of difference. Without it, the Boom factor is somewhat erratic. Some boom louder than others, and none as loud as with the cardboard. With the cardboard things are much more repeatable.
And, the baking soda is to prevent the cardboard from cooking the tomatos ;-)
Actually, that was my very first shot with this gun. After I fired that shot, I moved the gun into a safer direction. By safer, I mean in a direction where my wife would not kill me for taking out the entire garden with muzzle blast.
CW, if it wasn't for all the crapola in the background I would definitely enter it in the contest. I am going to have to try that picture again with a clearer background. I am thinking that picture with the muzzle blast peaking might have been a 1 in a million shot. Can you imagine that it was the first time that I fired it? I do have a way better camera now that might increase the odds of getting a muzzle flash without such luck. If the rain ever ends here I may try it again some time this week.
Rick