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I have the
replaceitem entity @a container.0 crossbow{display:{Name:"\"Paintball Cannon\"",Lore:["\"Put the fireworks in your offhand, and watch them go BOOM!\""]},Enchantments:[{id:quick_charge,lvl:3}]}"
command set up, but I do not know where to put the {unbreakable:1} tag to make the item...