How to measure the Distance of the Part?

You would do something like this:

local Bullet = script.Parent --Change this to the path of the bullet
local Active = true
coroutine.resume(coroutine.create(function()
    local StartPos = Bullet.Position
    while true do
        if (Bullet.Position - StartPos).magnitude > 100 then --If the distance between the StartPos and the Bullet is > 100...
            Active = false
            break
        end
        wait()
    end
end))
Bullet.Touched:connect(function(Obj)
    if Active then --If the bullet hasn't reached a range of > 100...
        --Do stuff
    elseif (not Active) then --If the bullet has reached a range of > 100...
        --Do stuff
    end
end)

EDIT This is a much simpler version that LightArceus suggested:

local Bullet = script.Parent --Change this to the path of the bullet
local StartPos = Bullet.Position
Bullet.Touched:connect(function(Obj)
    local BulletDist = (Bullet.Position - StartPos).magnitude
    if BulletDist > 100 then --If the bullet hasn't reached a range of > 100...
        --Do stuff
    elseif BulletDist <= 100 then --If the bullet has reached a range of > 100...
        --Do stuff
    end
end)