Bullet only goes on Z axis, not Z axis relevant to the gun?

I made a script, when you click, it shoots a bullet. Unfortunately, the bullet only goes on the Z axis, not the Z axis relevant to the gun! Why is that happening? Here is my script

wait(1)
script.Parent.Equipped:connect(function(mouse)
    mouse.Button1Down:connect(function()
--	local animation = script.Slash
--	local plyr = script.Parent.Parent.Name
--	local char = workspace:FindFirstChild(plyr)
--	local anim = char.Humanoid:LoadAnimation(animation)
--	anim:Play()
	local bullet = Instance.new("Part")
	bullet.FormFactor = "Custom"
	bullet.Size = Vector3.new(0.5,0.2,0.2)
	bullet.BrickColor = BrickColor.new("Toothpaste")
	bullet.BackSurface = "SmoothNoOutlines"
	bullet.BottomSurface = "SmoothNoOutlines"
	bullet.FrontSurface = "SmoothNoOutlines"
	bullet.LeftSurface = "SmoothNoOutlines"
	bullet.RightSurface = "SmoothNoOutlines"
	bullet.TopSurface = "SmoothNoOutlines"
	bullet.Velocity = Vector3.new(0,0,100)
	bullet.Parent = workspace
	bullet.Position = script.Parent.Handle.Position
	bullet.Transparency = 0
	bullet.CanCollide = true
	--local speed = script.Speed:clone()
	--speed.Parent = bullet
	local damage = script.DamageScript:clone()
	damage.Parent = bullet
	print("HALLAAA")
    end)
end)

Please help!!! Thank you!!!

If you want to use the relative z-axis of the handle, use script.Parent.Handle.CFrame.lookVector*100.