Well, you should probably first define "efficiency". Efficiency at damaging target's shield is meaningful, because flux used on the shooting end also builds flux on the receiving end. Efficiency at damaging armor/hull? Meaning comparing flux used to destroy X amount of armor/hull. Now this is getting quite abstract, though sure, you want the highest damage for the lowest flux cost. Right?

How you do achieve the highest damage at the lowest flux cost can not be extracted from a simple ratio built from (theoretical) damage and flux stats. That's why I highlighted some important stuff like turn rate, damage per volley, range, accuracy, volley time frame, and volley refire delay differences.

About (theoretical) damage and flux stats, let me show you, here:

Mark IX Autocannon has much higher per-projectile damage: 200 vs 100, so higher armor stripping power and longer shield overload duration.

Mark IX Autocannon has a 230 per-projectile flux cost, HAC has 100.

We have 200/230 for Mark IX vs 100/100 for HAC. Is it important? Yes. The most important? I don't think so. One should not ignore the other characteristics. Honestly I think this "theoretical weapon efficiency" is often overrated. And overall ship stats and actual battle situations will often (always?) make a bigger difference than single-weapon "theoretical efficiency".

(aren't large weapons supposed to be efficient anyways?).

Uhh. Is this written somewhere? I mean, sure, all 3 large HE guns are "efficient" (to various degrees, and one should be careful which definition of "efficient" he uses there). Should that apply to kinetic gun? I personally do not expect that.

What I would expect: the energy cost of pumping out heavier and faster projectiles is higher than the cost for lighter and slower projectiles. And I would also expect volley / burst of projectiles to affect the energy cost one way or the other depending on design.

Again, it's not a bad weapon, but it shouldn't have 1.15 efficiency for its performance.

1.15? Ok, I see you use flux/damage. Let's use damage/flux,

**Gauss has 0.58**HVD has 0.79

( Mjolnir has 0.8 )

**Mark IX has 0.87**( Heavy Mauler has 0.89 )

HAC has 1

( Hellbore has 1 )

( Hephaestus has 1 )

( Railgun has 1.11 )

**Storm Needler has 1.15**Arbalest has 1.25

( Light Needler has 1.25 )

Heavy Needler has 1.25

( Devastator has 2 )

HMG has 2.67

What does this magic number tells you about gun intended use and actual performance? Not much. In fact, if you are interested in

theoretical efficiency at dealing with target's shield, you could use (damage*2)/flux for kinetic guns. But even then, it would be far from telling the whole story.

And putting side by side kinetic / HE /energy guns as I've done above is not a good idea because one can't compare the ratio from, say, a Mark IX and an Hephaestus. Doing so would be meaningless without a narrowed scope. Using a narrowed scope such as

theoretical efficiency at dealing with target's shield, you may use (damage*2)/flux for Mark IX and damage/(flux*2) for Hephaestus. But... was the effort really necessary?

(don't get me started about comparing Hephaestus and Hellbore performance through such "efficiency" ratios)