A lot of people shoot a patched .690 diameter ball our of their 12 guage's. .715 diameter balls are also available and they will work either bare or with a thin patch.
Here's a link to Track of the Wolf's lead roundballs so you can see what is available.
https://www.trackofthewolf.com/list/Item.aspx/127/4
About the Diablo: These have been mentioned before on the Muzzleloading Forum and basically, they don't fit into the type of guns we talk about here.
This is a Traditional gun forum which basically means the guns we talk about are the old fashioned ones that were made before 1866.
Because it is built as a break open modern shotgun rather than a exposed hammer sidelock gun, talking about the gun itself isn't allowed.
We can talk about loads for good patterens and shooting roundballs out of the gun but photos of it aren't really welcome here. Ya, I know, some think we're strange but we like old fashioned guns.