Gun Emplacements vs Flyers • Mathammer

Here is some quick math for both weapons.

Assumptions
- BS4 & BS5
- Rounding up for everything (1st order effect)
- 5++ jink or invulnerable save for the flyers

I will only consider AV11 and AV12 because we want to know which gun is the best choice versus typical flyers.

Quad gun (twin linked) vs AV11 (BS4)
4 shots - 4 hits
50% chance to glance each hit, 33.3% chance to penetrate each hit
After saves are rolled:
Glancing = 4*0.5*0.67 -> 1+ glance
Penetrating = 4*0.33*0.67 -> 90% chance to penetrate (15% chance to assplodes)

Quad gun vs AV12 (BS4)
4 shots - 4 hits
33% chance to glance each hit, 17% chance to penetrate each hit
After saves are rolled:
Glancing = 4*0.33*0.67 -> 67% chance to glance
Penetrating = 4*0.17*0.67 -> 50% odds to inflict one penetrating hit (8% chance to assplodes)

I also assumed all shots hit, either due to twin linked or BS5 (another 1st order effect). This keeps the mathematics more simple with minimal effects to the final outcomes.


Icarus lascannon vs AV11 (BS4)
1 shots - 1 hit
83% chance to glance, 50% chance to penetrate
After saves are rolled:
Glancing = 0.83*0.67 -> 60% chance to glance
Penetrating = 0.50*0.67 -> 33% odds to penetrate (11% odds to assplodes)

Icarus lascannon vs AV12 (BS4)
1 shot - 1 hit
50% to glance, 33% to penetrate
After saves are rolled:
Glancing = 0.5*0.67 -> 33% chance to glance
Penetrating = 0.33*0.67 -> 22% odds to penetrate (7% odds to assplodes)

Conclusion
So neither is really all that great based upon the quick calculations presented above. I would say overall the quad gun is the better choice on straight odds. The Icarus lascannon has a better chance to get lucky though for the penetration due to its AP2 characteristic. I'd rather go with the straight odds.

If my math is correct then GW probably knew all along that neither gun emplacement is all that but they sell quite well because it's one of your better options and you gain the advantages of a fortification.