Fire:
    SHT2 A 3
    SHT2 A 7 A_FireShotgun2
    SHT2 B 7
    SHT2 C 7 A_CheckReload
    SHT2 D 7 A_OpenShotgun2
    SHT2 E 7
    SHT2 F 7 A_LoadShotgun2
    SHT2 G 6
    SHT2 H 6 A_CloseShotgun2
    SHT2 A 5 A_ReFire
    Goto Ready
    // unused states
    SHT2 B 7
    SHT2 A 3
    Goto Deselect

Quote from ZDoom Wiki
This function checks whether the player still has enough ammunition for another attack with the current weapon. If not, it starts a weapon change.

// [BB] Clients should only handle out of ammo weapon switches for themself, since
// they sometimes don't know the exact ammount of ammo the other players have. They
// are informed by the server of the weapon change anyway.
if ( (( NETWORK_GetState( ) == NETSTATE_CLIENT ) || ( CLIENTDEMO_IsPlaying( ))) && ( Owner->player - players != consoleplayer ))
    return false;

Quote from Edward-san

When and why was this code introduced?