How do I make a bullet marking script?

You may be familiar with the first two return values of FindPartOnRay: the part it hit and the point of intersection. There is a third value that you're ignoring, which is the surface normal. The surface normal is a one-stud long vector which is perpendicular to the face of the object.

Here's a picture I drew to illustrate what I'm talking about.

As you can see, no matter the angle of the ray, as long as they hit the same surface, their surface normals will be parallel. This is useful because this determines which direction the bullet hole will be facing.

Now assuming that you've already casted the ray, this is one way of making bullet holes:

--Creating the bullet hole which will be cloned every time a hole needs to be made
local bulletholepart = Instance.new("Part")
bulletholepart.FormFactor = "Custom"
bulletholepart.Size = Vector3.new(1, 1, 0.2)
bulletholepart.Anchored = true
bulletholepart.CanCollide = false
bulletholepart.Transparency = 1

local bulletholedecal = Instance.new("Decal", bulletholepart)
bulletholedecal.Face = "Front" -- you want it to be in front of the part for later
bulletholedecal.Texture = "http://www.roblox.com/asset/?id=2078626" --  a bullet hole decal I found 

function shoot()
    -- TO-DO: cast the ray
    local hit, pos, normal = game.Workspace:FindPartOnRay(ray)
    -- If I hit a wall, then
        local bullethole = bulletholepart:Clone()
        bullethole.Parent = game.Workspace
        bullethole.CFrame = CFrame.new(pos, pos + normal) -- the most important part: this sets the CFrame of the bullethole so that it's nearly flush with the wall. 
        game:GetService("Debris"):AddItem(bullethole, 10) -- automatically remove the bullet hole after 10 seconds
    -- TO-DO: do damage if it hits a player, etc.
end

Take note that you will have to add the bullet holes to the ignore list or else the bullet holes will stack with each other -- I'll leave that up to you.