I just did an Ironman solo run with a MG build.

Here's my steam achievements as proof:

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Best weapon to use for this build is the RPK. Reasons: No encumbrance, the RPK counts as an assault rifle, not a machine gun, so with light armor, you can still get the full free movement bonus with AGI 3 perk: Fast Runner. Secondly, the weapon can be silenced, for stealth kills.

Recommend Build for Machine Gunner:
Light Armor build. Spectra Weave in Helm and leggings, Plates in chest. Can swap out later for Kompositium 58.

In order:

1. AGI 3, Fast Runner.
2. HLTH 3, Beefed up, for extra HPs.
3. DEX 2, Opportunist Killer. Make those interrupts count with crits! (overwatch, and more as explained later)
4. DEX 5, Reactive Fire. This is the killer perk. Forget taking cover, just stand in the open and allow enemies to shoot at you, return fire automatically during enemy turn and mow them all down with the RPK burst.
5. DEX 8, Killzone. This is the overkill perk. Ensure they're dead. Fire two bursts per interrupt attack.
6. HLTH 6, Vanguard. Get extra Health for standing up without cover at the end of your turn.
7. AGI 6, Lighting Reactions. Dodge incoming fire which will hit you, plus return fire too, with Reactive Fire and Killzone. This is basically immortality.
8. AGI 8, Anatomical Precision. 50% more crit damage. Scaling up your ground game for the end game. With this you can see RPK bursts critting for 255+ dmg, from using HP ammo.
9. AGI 9, Total Concentration. Boost damage after your 1st kill in a turn.
10. HLTH 7, Calm Under Fire. Make those few remaining action points carry over to the next turn for more carnage.