You will have to find its exact name in the EQP file for the submarine, then "search" for it in the Library folder for it, or having the mod in front of you, search in its Library folder for the file that holds it, and adjust its sim file with the S3Ditor. I do not have that mod in front of me, so I don't know how he built the gun. He may have added a set of nodes to an existing guns_radar type file for a "new" gun, or he may have edited an existing 20mm gun, already in the game... Either way, you should be able to find it in the mod's folders, most likely in a Library folder. In the SIM file, you'll have to find the "wpn_Cannon" associated with that particular 20mm gun, and alter its "ammo_storage" settings for each of the different ammunitions available for it.
