Hellbringer Prime (stock) has PPCs in both arms. If you remove those PPCs, you'll notice you can now put on lower arm actuators. Great, isn't it? This is a part of the omnimech building ruleset, and Sarna backs it up. If you have ballistics or PPCs, no lower arm actuators. However, once those are removed, you're free to put on lower arm actuators again.
Relevant excerpt: "While they can be considered structural components, the lower-arm and hand actuators are themselves Pod-mounted and easily removed to provide additional Pod Space in the arms. However, the mounting points for them in the arms are also utilized by larger bore weaponry such as Gauss Rifles, Autocannons, and PPCs, so the lower-arm and hand must be removed to carry these weapon types on OmniMech arms, hampering the close-combat abilities of these configurations."
However, on many omnimechs, you simply can't do this. It's just broken. There's no other explanation.
List of all affected pods:
All Direwolf left arms
All Direwolf right arms (except the DWF-A)
Executioner right arm: b, d, c, ch
Executioner left arm: prime, a, b, ch, e
Warhawk right arm: prime, a, nanuq
All Warhawk left arm omnipods
Night Gyr right arm: prime, a, b, jk
Night Gyr left arm: prime, b, c, jk
Summoner right arm: fl, ml
Summoner left arm: fl, ml
All shadowcat right arm omnipods
Shadowcat left arm: a, b
Huntsman right arm: c
NOTE: Ebon Jag not listed because it is intentional that there are no lower arm actuators. It's locked like a Jagermech.
Example of how it should work:
Hellbringer Prime (stock) has PPCs in both arms. If you remove those PPCs, you'll notice you can now put on lower arm actuators. Great, isn't it? This is a part of the omnimech building ruleset, and Sarna backs it up. If you have ballistics or PPCs, no lower arm actuators. However, once those are removed, you're free to put on lower arm actuators again.
Link to relevant material: http://www.sarna.net/wiki/OmniMech
Relevant excerpt: "While they can be considered structural components, the lower-arm and hand actuators are themselves Pod-mounted and easily removed to provide additional Pod Space in the arms. However, the mounting points for them in the arms are also utilized by larger bore weaponry such as Gauss Rifles, Autocannons, and PPCs, so the lower-arm and hand must be removed to carry these weapon types on OmniMech arms, hampering the close-combat abilities of these configurations."
However, on many omnimechs, you simply can't do this. It's just broken. There's no other explanation.
List of all affected pods:
NOTE: Ebon Jag not listed because it is intentional that there are no lower arm actuators. It's locked like a Jagermech.