Switching weapon firing modes + burst fire mode

[Humility]

I want to create a weapon that uses two firing modes: automatic and burst fire.

Looking through, I think I would need to add this hook to the weapon code:

 function keyPressed (P, key)
if key == IN_WALK then
    if weaponFireMode == 1 then
      local weaponFireMode = 2
    else local weaponFireMode = 1
    end
end

hook.Add( "KeyPress", "switchFireMode", keyPressed ) 

I don't know too much LUA and the code above was half copied from the wiki and half created by myself. The lines with ( ? ) next to them are ones I'm pretty sure won't work.
I would've to declare the weaponFireMode to be either 1 or 2, 1 being auto and 2 being burst fire.

QUESTION: If I return else as nil ( nothing, basically ), will the weapon still fire or do I need to do something like:

 elseif key == IN_ATTACK then
( not sure what makes the weapon fire, tbh ) 
else
return nil 

Basically, do I need to account for all the possibilities before returning as nil? ( possibilities meaning reload, and stuff like that. )