In fact this is the correct way to use it, the Kubel gains more damage from you closing in on it, it will guaranteed win against an M3 that comes in closer than max range, rather than being an RNG fight. Why? Because the Kubel goes from 1.3 to 1.9 penetration, while m3 goes from 1.25 to 1.5, and the kubel also has some minor values like cooldown that are worse at range but better at short than on the m3. TL;DR: never close in with M3 unless kubel is not firing at you.
Actually, the Kubel loses damage from closing in on it, the Kubel fires shorter bursts at close range. At long range the Kubel has the burst advantage, firing longer bursts. At close range the Kubel fires shorter bursts than the M3, and gets wrecked for it. A long range fight is RNG, but a close range fight should go to the M3.
Edit: I just realised how redundant half the words in this post are. That's what happens when you don't pay attention to what you're writing, especially while watching TV.