You don't-- and here's why I think you don't.
I hate to say it but... (gulp) think in terms of the real world.
A weapon in the real world is a tool with several really simple design considerations.
Its purpose is to incapacitate the enemy, and it needs to do this as efficiently as possible.
Any energy spent over and above its intended purpose is an unnecessary inefficiency.
We're not necessarily just talking in terms of energy or resource expenditure with regards to the weapon and its ammunition (if any). You also have to consider things like its portability, its delivery systems, and its cost.
All of these things contribute to the "evolution" of weapons in such a way that they will always find the most efficient design.
Clearly, it's possible to build an advanced, futuristic weapon that does 4d6 damage. The problem is that 4d6 damage is "too much" with respect to the neutralization of human opponents. 2d6 works just as well. (Don't make me veer off into a discussion of hit points and what they represent-- accept it as truth that a 2d6 weapon is deemed sufficiently "deadly.")
So if Company A makes a weapon that does 4d6 damage, and Company B makes the exact same weapon that deals 2d6 damage in a more efficient way (less gunpowder in the ammo, lesser drain on the omega-zorper batteries, whatever) then the design that wins out will be the 2d6 version. It's as deadly as it needs to be.
And, of course, you always contend with Physics. The energy you get out of a weapon cannot be greater than the energy you put into it. Conserving this energy isn't important only to the weapons manufacturers of the day, its an imperative to the universal laws of physics.
Now, obviously, bigger, more bad-ass weapons exist for specialized purposes. You could, of course, choose to wade into every firefight with an RPG instead of a 9mm. But again, the point is that the 9mm is the more efficient design for killing human opponents, so that will be the weapon that is most likely to see widespread use.