The Weapon Workshop basically consists of a long workbench with several tools on the wall, with a Heavy Sniper in the middle on a weapon vice presumably undergoing the customization process. Racks of weapons and ammunition are shown on the wall to the left and right of the tool rack in the middle.
Grand Theft Auto Online
Selected weapons can be upgraded into Mk II variants, which offer an array of customizations not present on their standard models, including ammo types, unique attachments and aesthetic options (liveries and colors).
Restocking any special MKII ammunition (incendiary, FMJ, Armor Peircing, Explosive, and Hollow Point) requires a visit to the workshop while normal MKII Ammunition can be bought from the interaction menu.