In stock Space Empires IV, mounts are used to increase the damage and range of direct fire weapons when installed on larger ships, bases, and units. There are three types of mount available: Large, Heavy, and Massive; each confers a progressively larger damage multiplier (and a range and oddly enough an accuracy bonus with all but ships). The size, cost, supply usage, and hitpoints of the weapon are also increased, but the "bad" attributes are not increased as much as the "good", giving larger ships a significant advantage over smaller ships. (The most powerful mount, the Massive Base Mount, does 7x damage but only takes up 4x the normal space!)
Mounts can be modded using the CompEnhancement.txt file. Moddable attributes include range, accuracy, damage, hitpoints, size, and even shield generation. Mounts can be restricted to a specific type of weapon (or non-weapon components), a specific type of hull (ship, base, etc.) or a list of hull types, and/or specific families of component. There are also minimum and maximum hull tonnage limits, useful for making scale mounts. Not all mounts must "enhance" a component in all ways, however; some mods include a Gatling Mount which is smaller and does less damage but is more accurate, for instance.