Nuclear Throne Together allows you to define additional custom weapons. This is done by creating a yourweaponname.weapon.gml file in game's main or appdata directory and loading it via /loadwep yourweaponname. The file can #define the following scripts to be called by the game:

init

Executed when mod is loaded. Load resources here.

step

Executed by the player object when they are carrying the mod-weapon. argument0 will indicate whether the weapon is in primary slot.

weapon_name

Should return the weapon' name.

weapon_type

Should return the weapon' type (0..5).

weapon_auto

If defined, should return whether the weapon is automatic (true/false).

weapon_load

Should return the weapon' cooldown between shots, in frames.

weapon_cost

Should return the weapon' ammo cost per shot.

weapon_sprt

Should return the weapon' sprite.

Use sprite_add_weapon(base64, xorig, yorig) to auto-generate the "shine" from a single-frame sprite.

weapon_loadout

Can return the large weapon' sprite for use in loadout.

weapon_area

Should return the difficulty level at which the weapon should drop (-1 if it should not occur automatically).

You can add logic here to make weapons only drop on specific conditions

weapon_swap

Should return the weapon' swap sound.

weapon_melee

If defined, should return whether the weapon is melee (does "swings")

weapon_gold

If defined, should return whether the weapon is golden and should occur in Mansion.

weapon_laser_sight

If defined, should return whether the weapon has a laser sight or not. Enabled by default for bolt weapons and disabled for other types.

weapon_fire

Is called when the weapon is fired.

Should create projectile(s), do sounds, and call weapon_post(wkick, cameraShift, cameraShake) to shake/offset camera and weapon graphic accordingly.