MGs are not for Shadowrunners. Rather, the purpose on an LMG or MMG is as a dedicated vehicular or squad support weapon - covering fire for days. They are built to shoot and keep shooting until whatever you are fighting is gone. Portability, concealment, and ergonomics are not a real factor with this kind of weapon. Modern LMGs and MMGs use assault rifle rounds, typically the same rounds chambered in your lower-powered hi-capacity rifle for the LMG and your higher-powered low-capacity marksman's rifle for your MMG. Conversely, the HMG uses belts of sniper or anti-material rifle rounds.
Problem is, most SR combats I've seen are over within two combat turns, which I believe amounts to 6 seconds. Your average assault rifle will do the trick just fine for engagements much longer than this, and they really shine in comparison. The type of shadowrunner who would want to use an MG would be a troll, which makes sense given their large size. In this case, the large size and heavy weight of the weapon fit the ergonomics of the troll profile better than an assault rifle made for humans.
The big benefits of a machinegun aren't really covered by SR rules. First, machine guns are easily adapted to vehicles. In this edition of SR, however, all weapons are easily adapted to drones and other weapon mounts, so this aspect doesn't come into play in the rules. Second, modern machine guns are designed to operate jam-free, more-so than assault rifles. Well, jams only happen in SR when someone glitches and the GM decides that's how the story goes.
I would suggest that an MG could be fitted with a Smart Firing Platform and used remotely from a stand-off distance. Find a good spot to hide it and you've got squad support for up to 1500 meters away at the push of a button, with all the ammo you want to pack into it. This works great if your run is out in the open, or you've got a good view of the place you'll be doing your running. Of course, deckers can muck that up a bit...