The other Bandit If you've ever fired (or rather, tried to fire) an LMG from a standing position, you'd know that a heavier weapon does not for less recoil make.
During my service, we got to fire 200 7.62mm rounds from a German Rheinmetall MG-3 downrange at a full-size target (human adult equivalent), and the muzzle climb is so atrocious that I hit the target with two rounds. Scary as hell, yes. Effective as a suppressive weapon, yes. Accurate? Not only no, but hell no!
(In my defense, my first couple of rounds hit the piece of wood holding up the target and it fell over, but that still does not make up for a 2% accuracy ratio...)
Higher damage would be my best estimate. The same 7.62mm round impacts with approximately similar kinetic energy when fired from a shorter-barreled H&K G3 assault rifle as it does when fired from an MG-3; the MG has the advantage of proper bipod and tripod mounts, a barrel that can actually sustain 200 rounds of fire in a short time (20-25 rounds per second, in real life), a belt feed, and a quick-swap barrel. Despite that, the muzzle velocity and the mass of the round are still the major factors in terms of damage output.
The G3 would buckle from the heat of sustained fire long before the MG-3 did, but this is not reflected in SR5 game mechanics as barrel wear is not an issue.
To me, SR5 is more of a squad based system than all-out war. Most armies in the world do not make their soldiers carry emplacement-size MGs around with them, because these types of weapons are better suited for entrenched positions; the squad based weapons of modern warfare are almost exclusively lighter, smaller caliber, rapid fire weapons as opposed to the machine guns of World War I and II such as the MG-42.
A troll-sized metahuman might change the need for setup time, but it's still a glaringly obvious weapon. My Troll ganger-level character has had tons of fun running around with an Ingram Valiant when an Ares Alpha would have been "better", because with his relative skill a suppressive fire zone allowed him to a) scare the fuck out of the opposition, B) engage more targets, and C) introduce a dice pool penalty to the opposition. An AR can realistically do two (three in the case of one particular rifle) suppressive fire actions before reloading, while an MG can do 5. I think they are fine, statistically, for what they do...
Think of the Ingram Valiant more as the M249 chambered for 5.56mm, the Stoner-Ares M202 as the MG-42 (or more recent MG-3) or FN MAG (aka M240) chambered for 7.62mm, and the RPK HMG as the timeless Browning M2 heavy machine gun chambered for massive 12.7mm rounds, and you'll have an accurate representation of what I believe MGs are like in SR5.