@ Oliver Colt: According to the wiki, which may be out of date, the gatling's max range is 400m, and The arming time (the point at which the flak starts to do the most damage) is about 225 meters. So, that's a small 175 meter window for stripping the hull and then killing with the flak, especially at say, a charging pyramidion. According to the math, this loadout has a very narrow range of being at maximum effectiveness.
@Oliver Colt still: I just mean gunners who know the arc of the Hades and can consistently hit at medium range.
But the funny thing is, I stumble upon that 175 meter window a lot in a match. Usually when ships overshoot, are trying to run away, or are attacking an ally, such as a galleon. Plus, the junker is a slow ship, so that little bit of extra range the flak provides helps out when a ship is just outside of mortar range and you don't have the speed to fix it.
There's also the issue of actually hitting the target. When spinning about, decending, and acending, I'd rather have a gun that can hit in the narrow amount of time it takes to shoot the enemy while their armor is down. So, while the gat/flak has a narrow ~175m window to be the most effective it is always decent at the shorter ranges, and it's accessibility, fast projectile speed, and wider firing arcs allows a crew member of nearly any experience to hit the target.
Compared to the mortar, which is super effective in close, but looses out after the enemy leaves short range and requires a decent gunner. I think that flexibility and ease-of-use gives it to the flak instead of the the mortar most of the time in public matches on the junker.
Then again, I've got no clan and generally only play public games with random folk. I'll be the first to admit that a good gunner makes all the difference.