I think it comes down to how the pilot flies whichever ship is equipped with the Hwacha.
On a Galleon, Heavy Clip is a great supplemental ammo after Burst because you're never really sure at what range you'll be engaging. If paired with a Heavy Flak and a Gatling Gun, for example, the Hwacha on a Galleon can absolutely benefit from Heavy Clip when engaged at medium ranges.
The problem is... if there's a Gunner on that Galleon, he/she is most likely using those multiple ammo slots to take Greased/Lesmok/Burst to make the
Heavy Flak easier to fire and to address arming time. Because of this, the poor ol' Hwacha doesn't even get a chance to use Heavy Clip. "But what if you have Quad-Hwacha Galleon, Atruejedi?" Well, if you have even a single side stacked with more than one Hwacha, why bother engaging the enemy at anything more than point-blank range? So, again, Heavy Clip is rendered moot, even when carried by that Quad-Hwacha Galleon Gunner.
On a Spire, you have a situation similar to the one described in the previous paragraph. If your Spire pilot is taking a Hwacha, what are his/her other three light guns? Usually you'll end up with at least one Gatling (and more likely two), which is a closer range gun, even when paired with Lesmok... so, yes, while Heavy Clip can and will, on occasion, actually be loaded into a Hwacha on a Spire, it's a rare occurrence.
With a Goldfish, however, some pilots (myself included) insist on loading Burst because of the flanking potential and devastation the Burst-loaded Hwacha can have upon an unsuspecting enemy. There's no point in hitting them from afar with Heavy Clip if it's just going to alert them to your presence, fire fewer rockets, and have less AoE spread. It pays to wait until you're close enough to give them a punch in the gut with the Burst Hwacha.
Just another reason for Muse to #NerfBurst by removing the clip increase
That way, players might actually be encouraged to use other ammos in weapons like the Hwacha and Artemis.
Or, perhaps Heavy Clip needs an ever-so-slight buff by not reducing the clip size as much. Even a 5% change would help.