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).
For example, a rank 10 Heavy Caliber will reduce Flux Rifle's accuracy to 16.8 from 100.
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 is somewhat ineffective. Adding a maxed Heavy Caliber onto a maxed Serration yields only an additional 62% of the base shot damage. This is because the relative increase from 265% damage to 430% damage is not as significant as the increase from 100%. Players should exercise caution in comparing Heavy Caliber to more conventional mod choices, such as elemental damage.
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.
The accuracy penalty affects the Ignis and the Penta, but is essentially unnoticeable and is functionally used for pure damage.
There are currently no mods that counter the loss of accuracy from this mod.
When it was first released, Heavy Caliber increased recoil as well as damage. However the recoil penalty did not affect recoil-less weapons (Flux Rifle, Ignis, Synapse, Paris, Lanka, among others), effectively acting as another instance of Serration. This was confirmed to be unintentional in Warframe Prime Time - Episode 2 (9:40). The mod's effects were changed Update 10.4: it now gives an accuracy penalty instead of a recoil penalty.