How to make so gun ignore hats? (raycast gun) (i know how to add ignore list)

Well, if you want to add damage for hats (which is an accessory), then you have to add another if statement.

-- Example
if part.Parent.Parent:FindFirstChild("Humanoid") and part.Parent:IsA("Accessory") then

The above line will check whether the part it hit was a 'Accessory' or not. This is how your code might look :

local rem = game.ReplicatedStorage.hit
local temp = workspace.temp
local debris = game:GetService("Debris")
local headDmg = 20
local torsoDmg = 15
local otherDmg = 10
local dist1 = 50
local dist2 = 150
local dist1multi = 1.25
local dist2multi = 1.5

local function shoot(part,distance)
    if part then
    if part.Parent.Parent:FindFirstChild("Humanoid") and part.Parent:IsA("Accessory") then
        -- Your Code
    end
        if part.Parent:FindFirstChild('Humanoid') then
            local hum = part.Parent.Humanoid
            if part.Name == "Head" then --Head
                if distance >dist1 and distance <dist2 then
                    hum:TakeDamage(headDmg/dist1multi)
                elseif distance >dist2 then 
                    hum:TakeDamage(headDmg/dist2multi)
                else --normal distance
                    hum:TakeDamage(headDmg)
                end
            elseif part.Name == "Torso" then --Torso
                if distance >dist1 and distance <dist2 then
                    hum:TakeDamage(torsoDmg/dist1multi)
                elseif distance >dist2 then 
                    hum:TakeDamage(torsoDmg/dist2multi)
                else --normal distance
                    hum:TakeDamage(torsoDmg)
                end
            else --Legs, Arms
                if distance >dist1 and distance <dist2 then
                    hum:TakeDamage(otherDmg/dist1multi)
                elseif distance >dist2 then 
                    hum:TakeDamage(otherDmg/dist2multi)
                else --normal distance
                    hum:TakeDamage(otherDmg)
                end
            end
        end
    end
end

local function sfx(tool) --Gun shot sound
    local gssound = game.ServerStorage.sfx.gun_shot
    local sound = gssound:Clone()
    sound.Parent = tool
    sound.Playing = true
    debris:AddItem(sound,2.5)
end

rem.OnServerEvent:Connect(function(plr,mpos,opos,tool)
    local ray = Ray.new(opos,(mpos - opos).Unit * 250)
    local part, hit = workspace:FindPartOnRay(ray,plr.Character)
    local distance = (opos - hit).magnitude
    sfx(tool) --Gun shot sound

    --Damage
    shoot(part,distance)

    --Beam
    local part = Instance.new('Part',temp)
    part.Anchored = true
    part.CanCollide = false
    part.Color = Color3.fromRGB(255,180,0)
    part.Material = 'Neon'
    part.CFrame = CFrame.new(opos, hit) * CFrame.new(0, 0, -distance / 2)
    part.Size = Vector3.new(.1,.1,distance)

    debris:AddItem(part,.05)
end)

Lemme know if it helps!