How do I make these appear at a semi random place?

Okay so firstly i'd recommend indenting your code: but more importantly, when using CFrame, you can't just change the part's position -- you have to do part.CFrame.

CFrame.new() with only 3 parameters is basically the same as Vector3.new(), so you need to mention the components of the barrel's position (for example, Barrel.Position.Y)

math.random() can also only work with integers, so you have to give it a large range and then multiply it by a decimal. You also need to make it extend into the negatives to be evenly distributed on the X axis.

Here's how that looks in your script:

function onClicked()
    local Cannon = script.Parent
    local Barrel = Cannon.Barrel
    local Fireball = game.ServerStorage.FireBall
    local fireballCopy = Fireball:Clone()
    fireballCopy.Parent = game.Workspace
    fireballCopy.CFrame = CFrame.new(Barrel.Position.X + math.random(-20, 20)*.01, Barrel.Position.Y, Barrel.Position.Z)
    fireballCopy.Velocity = Vector3.new(100,0,0)
end

script.Parent.Parent.Part.ClickDetector.MouseClick:connect(onClicked)

I would probably just do this:

fireballCopy.Position = Vector3.new(math.random(-1,1),0,0)

Change those numbers to match the barrel's coordinates, but for the X-Value, have one random number be increased slightly, and the other decreased slightly.

For example:

The barrel's position is (15,4,-60), so the fireball's position will be (math.random(14,16),4,-60)