Here's what I did.
Start up the Goblin Editor.
Click File-Open
(Browse to game installation) data/objects/guns/
Choose a guns GR2 file.
Then Merge that guns corresponding .sim & .cam files.
I then double clicked on "wpn_Cannon @
your chosen gun" under Project Tree, this will open the "Edit Behaviors" window.
Under "Existing Behaviors" click on "wpn_cannon".
Expand the "ammo storage" option.
Change the "amount" value for AP,HE & AA, enter 0 for nothing.
I haven't got the star shells to work yet.