Heavy SRM
The Heavy SRM launchers are less-common alternatives to the standard SRM launchers. The Heavy SRM actually predates the now more-common variant, introduced around the same time as the LRM. With a missile weight twice that of the LRM, the Heavy SRM packs a warhead 3x more potent than the LRM's at respectable ranges. However, the Heavy SRM launchers were on the heavy side, which usually meant significant sacrifices had to be made when mounting them as secondary weapons.
The "Standard" SRM was developed later to miniaturize the launcher, so the SRM could be use as a secondary weapon to supplement autocannons and rifle cannons with few sacrifices. The new SRM launcher's weight was halved, its bulk reduced, and its ammunition bin expanded through the use of a lighter missile at the cost of only some range and damage. This was considered a worthwhile trade, as the new SRM became popular and common enough to become considered the "Standard" SRM launcher, while the original older SRM launcher became the "Heavy" SRM launcher.
While the Heavy SRM is not "rare" in any sense of the word, the Standard SRM is the clear favorite on the battlefield. The Heavy SRM is mostly found on specialized missile boats and troop garrisons where the heavier weight of the launcher and ammo is not as large of a concern. Great House military forces on the offensive and mercenaries generally prefer the Standard SRM for most scenarios, with the notable exception of the Lyran Commonwealth which field a disproportionately large number of HSRM-equipped designs.
HSRM2 HSRM4 HSRM6
Dmg/missile 3 3 3
Range 4/8/12 4/8/12 4/8/12
Heat 2 3 4
Shots/ton 30 15 10
Tons 2 4 6
Crits 1 2 3