In comparison to Serration, Heavy Caliber costs two more mod power for the same damage buff despite the accuracy reduction.
Instead of reducing the accuracy by 5% per rank, each rank increases spread by a specific percent per rank depending on the weapon. This effectively reduces accuracy, but may reduce it by greater than 5% per rank (according to the Arsenal UI).
Damage increase of Heavy Caliber stacks additively with that of Serration. With Serration and Heavy Caliber both maxed, you can reach +330% base damage in total, before adding Elemental and Faction based damage mods.
Since the two stack additively, combining the two can be cost ineffective when weighed against mod point drain. Adding a maxed Heavy Caliber onto a maxed Serration yields only an additional 62% to your base shot damage. This is because the relative increase from 265% damage to 430% damage is not as significant as the increase from 100% to 265%.
However, this is still a base damage increase, so elemental mods multiply with, rather than add to, Heavy Caliber's effect.
Accuracy will suffer on weapons without 100% accuracy, affecting the actual bullet path. This makes even accurate guns like the Soma spread bullets considerably at maximum rank.
Not recommended for the Miter as the projectiles will fly very far from the reticle. Strangely this does not apply for the Panthera despite the similarities between the two.
Currently, the only mod that can reduce Heavy Caliber's negative effects is Guided Ordinance, and only when you successfully hit an enemy.