Metatable proposal

local wep3 = {} -- our "Entity", imagine userdata here

-- helper, ignore
local function set_lookup_fallback(what,towhere)

    local m = getmetatable(what)
    if not m then
        m = {}
        setmetatable(what,m)
    end
    m.__index = towhere

end

-- Weapon and Entity metatable functions here like normally
local WeaponFuncs={}
local EntityFuncs={}

-- examples
function WeaponFuncs:IsValid() print"COMMON (WEP)" assert(self==wep3)  return true end
function EntityFuncs:IsValid() print"COMMON (ENT)" assert(self==wep3)  return true end
function EntityFuncs:ent    () print"ENT (ENT)" end
function WeaponFuncs:wep    () print"WEP (WEP)" end

-- weapon looks into entity metatable if it cant find something
set_lookup_fallback(WeaponFuncs,EntityFuncs)

-- single entity specific
-- these tables would need to be created for each entity
    local wep3_data = {} -- this saves entity specific data

    set_lookup_fallback(wep3_data,WeaponFuncs) -- not found, lookup Weapon -> Entity

    local wep3_meta = {
        __newindex=wep3_data,
        __index=wep3_data,
        __tostring=function() return "Weapon[3]" end,
        --etc
        }
    setmetatable(wep3,wep3_meta)

-- testing
    wep3.asd = 123
    assert(wep3_data.asd==wep3.asd and wep3.asd==123,"data vanished")
    assert(isfunction(wep3.IsValid),"wep3.IsValid not a function")
    print("IsValid",wep3:IsValid())
    wep3:wep()
    wep3:ent()
    print("tostring",tostring(wep3))