I think I found the reason why the light cannon is doing worse in comparison to the HMG when the light cannon has only one damage upgrade. If the normal calculation would give a result that a weapon would deal no damage, it deals a minimum damage of one-third of its base damage. For example, if the weapon has a damage per round of 30 after modifier but the enemy has an armour of 50 the weapon will deal a damage of 10 per round. And that's the fact for the HMG against the NP hard structures. Machine guns have a modifier of 25 against hard structures. So the HMG would deal a damage of 8 against a hard structure with an armour of 15. The result would be no damage and in this case, the one-third-rule takes place and the HMG deals a damage of 10 per round. The light cannon with one damage upgrade deals a damage of 11 per round. And because of the higher ROF of the HMG, the light cannon is remarkable worse against hard structures what a better accuracy can't compensate. The light cannon is only slightly better against the NP units. So for insane difficulty, I suggest we give the light cannon still two damage upgrades but activate the ground reinforcements of the NP. But only for insane difficulty, not for the other difficulties.
Edit: The one-third-rule does not only takes place if the result would be no damage but also if the calculated damage is lower than one-third of the base damage. This means a weapon with a base damage of 30 is always doing a minimum damage of 10 per round. Only if the calculated damage is higher than 10 the calculated damage is used.
Interesting I wondered why light cannons where doing worse then heavy machine-guns even though they had comparable values, I guess that would also explain why flamers where doing next to no damage against new paradigm structure because it's damage was so low that ultimately it was doing zero damage by the time you factors in armour and modifiers and was therefore invoking the one-third-rule, but since the damage of flamers was so low it was only doing like 8 damage.
I guess the obvious solution then is to slightly increase the damage and / or modifiers of weapons that would otherwise deal no damage when you factor in enemy armour and modifiers thereby avoiding invoking the one-third-rule in the first place.
which means increasing the damage and / or modifiers on heavy machine-guns and flamers and light cannons.
so for the flamer and machine-guns I'd increase the hard modifier to say 50 and maybe increase the damage by 10 as well because there hard points seem to have armour of 15 so for the heavy machine gun that would be
40 damage - 15 armour = 25
with a hard modifier of 50 that 25 / 2 = 12.5 damage per shot
12.5 x 107 = 1337.5 damage per min vs there hard points
for the flamer that would be
46 damage - 15 = 31
with a hard modifier of 50 that's 31 / 2 = 15.5 damage per shot not including the dot
15.5 x 34 = 527 damage per min not including the dot vs there hard points
and for the light cannon I'd increase the damage back to 30 but decrease the rate of fire to 45 then with 2 damage upgrades that would be
48 damage - 15 = 33 damage per shot since all rounder has a hard modifier of 100
33 x 45 = 1485 damage per min vs there hard points
with its current values its 32 - 15 = 17 damage per shot since all rounder has a hard modifier of 100 17 x 60 = 1020 damage per min vs there hard points.