One of the old container city videos showed a turret firing at an enemy up to 30 feet.
There is no guarantee that the three turret abilities are progressively stronger versions of each other. The Chaingun turret could be specialized to take down infantry, sacrificing range for RoF and accuracy whereas the Medium turret could be a slower, firing long range support turret and the Light would be a front-line, general use turret.
However I am just entertaining myself and they probably are progressively better versions. Nevermind.