Considering the setting, it's not CoD, but movie, the Terminator (perk tree icons even) that's the inspiration and what kind of game play game is trying to enable here.
I don't see a reason why HMG:s shouldn't benefit from skill level bonuses and perks related to LMG's. How bonuses are applied 100% to 75% effective I don't have an opinion, but I think it it would be good to see what is LMG at first.
Some deconstruction starting with LMG's.
In Real Life: If you are firing any machine gun from non fixed position, something has already gone awfully wrong.
Even LMG's are relatively heavy, they are also relatively big and so if you aren't using supports, you need help aiming by rotating your body. Then you are firing from the hip, so sights are out of question and wouldn't matter anyways because since you aren't using supports, they are not taking any of the recoil. In the end you can't hit a damn anything really. Can still be try to fill fire support role theoretically, but that really won't last long though, while standing and being a huge stupid target with a clumsy weapon to use like that. Like I said, something has gone awfully wrong if that happens.
In Game: We can this Darwin award winning scenario as we have possibility to create our own Arnold via cybernetics such as; subdermal armor, pain editor, synaptic signal optimizer. Then there is Body primary stat (body HMG movement speed) and Body related skill bonuses. Athletics skill level, (max health, armor) and perk options such as; invincible, steel shell, indestructible. Then there is even more Annihilation level progression and perks. Skill level related bonuses in Annihilation tree were far more effective than I expected. Perks reducing recoil are useful too, but I found Biathlete very useful, though I mainly used shotgun as firearm.
There is so much synergy in perks, that I won't get in more detail. Lots of
Unknow variables:
Athletics skill: Max skill level unattainable by normal game play, but only via Berserker glitch. Most players probably don't even know about that glitch.
Dog of War Perk: Didn't work at all,
- None of the perks perks that should enable health regeneration in combat worked for me at all.
Thoughts: LMG builds aren't probably be intended to be top DPS dealers, but have a lot staying power and keep elimination going despite being getting hit, being very durable Arnold rather than Terminator. IMO idea works as the next step would mean becoming Adam Smasher.
Broken attribute level progression and broken perks make creating build that very difficult, as Athletics is broken, also perk points available from skill levels are unattainable for most players.
Depending from character and difficulty level, intended(?) playstyle cannot be achieved due broken heath regeneration perks. Medium DPS dealers MaxDoc doesn't substitute, as using it reduces DPS.
Before related perks are fixed, IMO shotgun leaves LMG's mostly redundant as movement bonuses from perks and skills simply favor Close Quarters Combat scenario for high body builds. Farther, using melee and using MaxDoc synergizes with CqC fairly well for high body builds.
LMG's can be important for various reasons, but possible rebalancing should start after fixing perks as for now, creating a built that benefits from all skill level bonuses and perks that synergize, is impossible.