A few points I would like to make:
For one,
I don't like you guys comparing the Banshee and the Artemis to the Flak.
While the Flak does have more killing potential, these guns have uses in disabling which means that they have uses for outside the armour break.
As such, I don't think it is very nice to compare it to the light carronade either. Light Carronade can pop balloons in two clips but I think it still isn't fair to compare it to the light Flak. Mainly because popping balloons can disrupt arcs, make a ship vulnerable to Terrain, make the Hull more exposed, and separate enemy ships.. Even damaging balloons can cause a decrease in vertical mobility. The Flak can only kill and there is plenty of time for an enemy to incapacitate/ disable you in the time of that second clip.
It is viable and necessary to compare the light flak to a banshee or an artemis, exactly because of their multi-role purposes and varied circumstances of usage. At the end of the day, when you are creating a ship load-out, and you are contemplating which gun you want that can damage hull, you have a wide range of choices simply because the light gun slot is not restricting you with which gun you can place where - you have an unlimited range of options to go for, so you need to weigh all the options before you and pick for yourself which gun will be best suiting your case.
That is why you ultimately NEED to consider and put into equation all other 'semi-explosive' guns.
I don't think I like the idea that a gun that is only useful for when the armor is broken can't kill a ship in one clip when we have two guns that have that ability. It means that we have a gun that is only good for killing ships and can't do it faster than two other guns. I find it annoying that a gun that is useless for a vitally important portion of a fight will have to have the help of other guns that speed up the killing process.
You have a gun that is useless until the short window of opportunity in the armour break, only useful at the small window of mid-range where engagements are less common, and you can't kill a ship faster than other guns that are useless until armour break.
The reason why the mortar is able to kill a ship with one clip is because the extremely limited range it has, due to jitter and low projectile speed I would place it as not reliable for killing on ranges above 200m.
The reason why the heavy flak is able to kill a ship with one clip is because the extremely low projectile speed it has for a gun with such range, and quite arcing shots over distance. At long ranges, the bullet travels so much time in the air, that it gives you an impressively small window for your shots to land if you are trying to time the armor break. What most people end up doing is to just spam it all the time and hope for the best. All-in-all, the heavy flak's ability to one-clip-kill ships I will place as reliable only at about below 1-1.2k range, and around 1.5k range for more experienced gunners. Factor in that a lot of maps will not give you the opportunity to snipe someone all the way from 1.5k until he's right in your face, and you get shorter effective ranges, with the enemy being at about 500m already and closing in fast - it almost always spells failure for a long range heavy flak build.
Switching to the flak, first of all, the flak is able to one-clip-kill Mobula, Pyramidion and Junkers. With placing significant damage to kill any other ship with the second clip.
With an arming time of 150m that can be manipulated a bit with ammo types, an a max range of 1000m, I will place the effective range of the flak from 200m to 850m. Those are 650m of range it is effective on, compared to 200m of range for the mortar, which explains why taking a mortar has to be a lot more rewarding due to the higher limitations. As to the heavy flak's effective range - around 500m~1200m which is pretty much the same range window as the light flak's give or take 100m, but the heavy flak needs to be able to perform those one clip kills due to the relatively hard nature of firing the weapon.
There needs to be a reward factor when using a weapon as hard to use as the heavy flak, or as range limiting as the mortar. The light flak is an inbreed, both possessing the higher range window of the heavy flak, and ease of use of the mortar - a combination of traits that make the light flak the go-to weapon if it could one-clip kill every ship, there needs to be a penalty for it and that is the limiting factor.