I was trying to add the AK Ball magazine to the OPFOR Arsenal.
When adding a new entry to the Arsenal Configs Arsenal Items the default value for the new entries Item Type is Rifle, which fits my use case.
If I do not touch the Item Type and leave it as if, the Item will not appear in the Arsenal
If I now select the Item Type to be Rifle, same as it was before, it will appear in the Arsenal
Note the Item Type text now being in bold and white