When firing certain items from the Thing Flinger, certain things will happen.
Weapons/Tools: When weapons are shot from the Thing Flinger, they will do the amount of damage that the weapon itself does. (Example: You launch a Terra Blade doing 100 damage, the thrown Terra Blade will do 100 damage.) Same applies to anything else that does damage, like ammo.
Potions: If a potion/flask is thrown from the Flinger, it will give whatever it came in contact the corresponding buff/debuff would be applied to the enemy. This also means that you could restore player health/mana with those potions, but they would still get health/mana sickness. However, the buff/debuff would only last for five seconds. This also applies to Cursed Flame/Ichor spell tomes. Note that if you throw pure Ichor/Hellstone/other ingredients to potions, that they will NOT give a debuff.
Explosives: When explosives of any type are thrown, they will do 50 damage and explode/destroy blocks on impact. Any explosives with effects (sticky, bouncy, etc.) will have their effects for 3 seconds and then explode. Explosives can’t be retrieved after being flung. Items that count as explosives: All Grenades, Bombs, Dynamites, Fish Bombs, Explosives, Detonators, Beenades, Happy Grenades, Land Mines, and Holy Hand Grenades (which would do 100 damage, even though it might be removed.)
Thing Flinger: Flinging a Thing Flinger will cause another piece of ammunition to launch from the launched Thing Flinger, but will only do 1/2 of the Damage. If a Thing Flinger is thrown from a thrown Thing Flinger, it will NOT fling another item.
Everything Else: Damage depends on the rarity. Rarity damages: Gray=10, White=15, Blue=20, Green=30, Orange=35, Light Red=40, Pink=50, Light Purple=55, Lime=60, Yellow=70, Cyan=85, Red=100, Purple= 120, Amber=40, Rainbow=90.