A_JumpIfNoAmmo
From ZDoom Wiki
A_JumpIfNoAmmo (int offset)
A_JumpIfNoAmmo (str "state")
This function can only be used with weapons. It jumps if the carrying player doesn't have enough ammunition for one attack of the currently used attack mode (defaulting to primary fire if called outside of a fire state).
Offset is the number of frames to jump ahead when there is no ammo. State is what state to jump to when you're out of ammo.
Examples
States
{
Fire:
CISG A 0 A_JumpIfNoAmmo("Reload")
CISG B 1 BRIGHT A_GunFlash
TNT1 A 0 A_FireCustomMissile("PistolShellcasingSpawn",0,0,-6,2)
CISG B 1 A_FireBullets(3,2,-1,9.2,"BULLETHIT",1,0)
CISG C 2
CISG D 2
CISG E 2
Goto Ready