2 separate thoughts here:
I think the way to reel back in the mortar is to decrease the speed boost from lesmok. The 80% is what's causing the problem, further ammo reduction seems unnecessary.
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The reason why the light flak has come out of favor is 2 fold.
The flak was used heavily in competitive play and emulated in pubs due to it's pinpoint precision killing power with heavy clip. Though a mortar had way more power and was better in close range a flak was practically infallible. If you buffed it and used heavy clip, any ship besides a goldfish or galleon would evaporate when the armor dropped. Well timed light flaks could actually reload and finish galleons as well before they fully rebuilt and goldies easily had their armor brought back down.
Before the heavy clip was changed, light mortar lost its jitter. This started the transition where about 20-30% of players actually started preferring mortar because it became possible to hit with. Many older competitive teams stuck with the flak for its precision but newer teams started experimenting with the raw power of the mortar.
Then heavy clip caused light flak to lose a shot. That was, as Sammy described, the nail in the coffin. The flak invariably became obsolete since in a 1v1 brawl even if the flak ship dropped the mortar ships armor; it couldn't secure the kill, so the mortar ship would rebuild and kill the flak ship first even if the flak ship had the advantage.
So precision vs raw power doesn't become a debate if precision can't secure a kill with 1 clip. So in a way heavy clip light flak has the exact same problem as lesmok heavy flak. Guns that used to be very good due to their accuracy, lost their competitive edge due to the inability to 1 clip kill.