Part not being positioned by script?

I'm using two beams to simulate the 'trail' of a mortar's shell. One beam goes from the barrel to a dummy part that houses the second beam, and the dummy part attaches itself to the unit it's firing at. The problem is that the dummy part when created positions itself at 0, 0, 0 even though I'm using Part.Position to position it half-way between the mortar and the enemy.

Here is the code in question (I've cut off the rest of the script for readability).

Is the problem that I'm using an equation? Or is it because I should use CFrame?

local maxTrajectory = Instance.new("Part", workspace)
maxTrajectory.Size = Vector3.new(0.05, 0.05, 0.05)
maxTrajectory.Position = Vector3.new((v.Torso.Position + thisUnit.Torso.Position)/2 + Vector3.new(0, 6, 0)) -- i average the positions of the enemy and the unit itself to find the midpoint here and then add 6 to the y-coordinate otherwise the position is level
maxTrajectory.Transparency = 1
maxTrajectory.CanCollide = false
maxTrajectory.Anchored = true
local maxBeam = Instance.new("Beam", maxTrajectory)
maxBeam.CurveSize0 = 2
maxBeam.CurveSize1 = 0
maxBeam.Width0 = 0.05
maxBeam.Width1 = 0.05
maxBeam.Segments = 100
maxBeam.FaceCamera = true

You need to put in X, Y and Z. You only put X.