Use black powder or one of the BP substitutes.
You'll need an 'over powder wad called a 'Nitro Card Wad' (hard dense cardboard).
1/8" thickness is a common size. Not always available in the large guages but 8ga is something they should have even in smaller quantitys.
On top of the Nitro card, generally is placed a filler wad of a less dense material.
Some were made from felt, cork,, today most are a semi dense fibre material (think dense ceiling tile material).
These are wax coated on the edges usually.
These wads need to fit firmly inside the case so the powder burns correctly and gives you the loud report instead of a blooper as they're called.
Sometimes wax is used to seal over the top of the fibre wad to water proof the rounds for storage.
Go to Circle Fly Wads:
Products
Look around on their web sight. Lots of info including dimensions of case and bore sizes for the different gauges/calibers.
Loading info, and other general information. You can email them with any questions you have and from my experience in the past they've been very helpful.
Blanks can be made with nothing more than densely packed newsprint on top of the BP charge. The tighter packed the better.
One problem with these (and they do work OK) is the fire hazzard they pose.
Another is the litter of shreds of unwinding newsprint out in front of your cannon. Particulary untidy over the water!
*** Be as careful with blank rounds as bulleted & shot rounds. The wads used are as much of a lethal projectile out to certain ranges as bullets and shot. ***
