If you want to play with black power, 6 drams is considered a traditional BP blank. To be perfectly honest, it is louder than a live round... 5 drams is fine.
Keep in mind that BP needs to be cleaned with water, not modern solvents. If you are using a pump or semi-auto, I would suggest that you do not do it. If you have a break-open single or double, ... heck... they were made originally to shoot BP so they come apart nicely and you can clean the barrel in the kitchen sink. Find some "Ballistol" to use after the water in place of whatever typical gun oil you would use.
I cut down 12 g hulls to 2.5 inches, load with 5 drams BP, cover with an overpowder card, and roll crimp. The shorter hulll makes for easy identification.