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 and Heavy Caliber costs 2 more mod capacity for the same bonus, combining the two is slightly less efficient in terms of damage per capacity cost. Maxed Serration grants +11.79% bonus damage per 1 mod capacity, while maxed Heavy Caliber grants +10.31% bonus damage per mod capacity. Combining the two, Serration and Heavy Caliber grant +11% bonus damage per 1 mod capacity, which is +0.79% less bonus damage per capacity than Serration alone.
The bonus damage from using Heavy Caliber with Serration is double that of using Serration by itself, though it comes at the cost of an additional mod slot and Heavy Caliber's slightly higher "base" drain of 6.
For example if base damage is 100 then Serration adds 165% bonus damage and Heavy Caliber adds an additional 165% bonus damage, for a total of 430 damage. Thus, bonus damage is doubled for roughly double (+114%) the drain cost.
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.