Hey, who remembers the old CEV (Combat Engineer Vehicle)?
Look how SHORT, STUBBY, and WIDE that gun is. That's it. That's all she had. THAT is your doggone AC/20. Unless you want the gun barrel to be 6-7 meters long and weight 15,000 pounds by itself, that's what you get. It has a VERY short range, therefore. It was actually designed to be a short standoff demolition device--it fired heavy, high-explosive rounds into countermobility structures in order to clear the way for vehicles to pass through them. It's what combat engineers do.
Now, consider:
Look at THAT barrel! Longer, narrower. It fires a smaller projectile. It fires it farther, more accurately. But that round doesn't carry the same energy, necessarily, because it has far smaller mass, and less space into which to pack explosives. Et cetera. That's more of an AC/5. It doesn't blow up berms and dunes and hills. It just rings other tanks' bells. With sabots or shaped charges, but anyhow...
Gee, fast-forward to early 3050, and so little has changed. It's comforting, in a way.
There IS something with massive damage, like and AC/20, with amazing long range, like an AC/2. But it has a lot more of both, and never sees what it shoots. I've heard RUMOR that field artillery is in the future plans for this game. Until then, NO, you do NOT get a God Gun. Spot well for your LRM boats, and have a nice year. I have a feeling about 3050...