Is it like that: you give pixie dust with ammo index “gel” and then the weapon with ammo use of “gel”?
yep. It’s what I used to make hellstone, Soul of Fright, and pink slime blocks ammunition.
/sitem -i “Pixie Dust” -ammo (any number)
/sitem -i “(any weapon)” -useammo (same number as above)
Of course, Pixie Dust doesn’t actually have a projectile, so you’ll need to modify its projectile (not the weapon’s) to make the weapon fire anything when using it as ammo. You can also modify the Pixie Dust’s damage and such. This can be used with just about any item, although some items (seemingly unstackable ones like weapons?) won’t be consumed when used as ammo.