This is not a comment about low/medium level matches where you can do what you like and most likely you're going to get away with it.
It takes a number of very high level matches, players that focus on what they want to do, do it right, and a bit of ingenuity on your side to prove or disprove some sort of theory.
This answer is not a proof that heavy clip is useless in hwacha. Maybe you will find, one day, a repeteable situation where it is. More likely the game will die or get completely rebalanced, still.
Last of all, some questions can be answered only by statistics in specific situations and it's damn difficult to provide those.
It's an answer based on competitive and casual experience, but also lengthy discussions. And I probably forgot about a number of situations.
As of the time being, the only situation where heavy clip can be negotiated, is when you need to use hwacha as a close range defense weapon, rather than something for offense.
It may be hwacha on long range side of a sniping galleon, where gunner takes over the hwacha while some evil mobula gets into dangerous arming time of lumberjack. But hey, in most such situations, the huge majority of legit pilots will put another long range gun for better support and let's hope we can turn to close range side when we have to. If one of enemy ships is close range, it's already better to bring burst for engineer in the first place.
It may be hwacha on a long range spire that pretends to be a mobula (double art plus hades/merc). In most matches with long range ships enemy pilots will instantly switch to double mercury pyras, cheese pyras, double hades pyras, mercury mobulas, hades mobulas, meta galleons etc. while congratulating themselves the knowledge of how to bring proper ships in proper situations against proper enemies or because they wanted to. Assuming this situation didn't actually happen and we have some sort of hope that it's going to work, we'll have the gunner shooting one of the light guns usually from long range. But, since the hwacha sits idle, he might as well fire a heavy clip of hwacha occasionally. The problem is, either he's going to miss, or the enemy will intentionally dodge, or the hwacha will hit and not do anything, because it's heavy clip and not burst.
Finally, there may be a super weird situation where you need to stay at range for some reason (like first enemy just died, ally finishing off an enemy, we're waiting at their spawn), or charge to enemy spawn in a long range map where you just died and hope your ally will survive. At such times, you will have time to reload the hwacha before getting close to the enemy, so there's a tiny little little chance that it's going to help something, even on a hwachafish. Most likely though, you'll be wasting a gunner ammo slot instead of using something that has low probability of being useful, but still better than long range heavy clip hwacha (such as heatsink).
Otherwise, the most mistaken notion is shooting the charging enemies because they're still far away.
You're wasting your time getting close to them, not giving enough fire support to your ally. This also happens regularly in so-called high level matches.
Or they're going to get close much sooner than expected and you will die because no reloaded burst hwacha.
Or they're not coming any time soon anyway and you might as well be shooting a harpoon at a passing skywhale instead.
